diff --git a/include/cantera/base/AnyMap.h b/include/cantera/base/AnyMap.h
index 27f4a17383..5ad8a66cee 100644
--- a/include/cantera/base/AnyMap.h
+++ b/include/cantera/base/AnyMap.h
@@ -25,6 +25,36 @@ class any;
 namespace Cantera
 {
 
+//! Base class defining common data possessed by both AnyMap and AnyValue
+//! objects.
+class AnyBase {
+public:
+    AnyBase();
+    virtual ~AnyBase() {};
+
+    //! For values which are derived from an input file, set the line and column
+    //! of this value in that file. Used for providing context for some error
+    //! messages.
+    void setLoc(int line, int column);
+
+    //! Get a value from the metadata applicable to the AnyMap tree containing
+    //! this node.
+    const AnyValue& getMetadata(const std::string& key) const;
+
+protected:
+    //! Line where this node occurs in the input file
+    int m_line;
+
+    //! Column where this node occurs in the input file
+    int m_column;
+
+    //! Metadata relevant to an entire AnyMap tree, such as information about
+    // the input file used to create it
+    shared_ptr<AnyMap> m_metadata;
+
+    friend class InputFileError;
+};
+
 class AnyMap;
 
 //! A wrapper for a variable whose type is determined at runtime
@@ -43,7 +73,7 @@ class AnyMap;
  * - `std::string`
  * - `std::vector` of any of the above
  */
-class AnyValue
+class AnyValue : public AnyBase
 {
 public:
     AnyValue();
@@ -68,15 +98,6 @@ class AnyValue
     //! providing informative error messages in class InputFileError.
     void setKey(const std::string& key);
 
-    //! For values which are derived from an input file, set the line and column
-    //! of this value in that file. Used for providing context for some error
-    //! messages.
-    void setLoc(int line, int column);
-
-    //! Get a value from the metadata applicable to the AnyMap tree containing
-    //! this AnyValue.
-    const AnyValue& getMetadata(const std::string& key) const;
-
     //! Propagate metadata to any child elements
     void propagateMetadata(shared_ptr<AnyMap>& file);
 
@@ -209,16 +230,6 @@ class AnyValue
     template<class T>
     void checkSize(const std::vector<T>& v, size_t nMin, size_t nMax) const;
 
-    //! Line where this value occurs in the input file
-    int m_line;
-
-    //! Column where this value occurs in the input file
-    int m_column;
-
-    //! Metadata relevant to an entire AnyMap tree, such as information about
-    // the input file used to create it
-    shared_ptr<AnyMap> m_metadata;
-
     //! Key of this value in a parent `AnyMap`
     std::string m_key;
 
@@ -247,8 +258,6 @@ class AnyValue
     static bool vector2_eq(const boost::any& lhs, const boost::any& rhs);
 
     mutable Comparer m_equals;
-
-    friend class InputFileError;
 };
 
 //! Implicit conversion to vector<AnyValue>
@@ -347,7 +356,7 @@ std::vector<AnyMap>& AnyValue::asVector<AnyMap>(size_t nMin, size_t nMax);
  * }
  * ```
  */
-class AnyMap
+class AnyMap : public AnyBase
 {
 public:
     AnyMap(): m_units() {};
@@ -385,19 +394,10 @@ class AnyMap
     //! messages
     std::string keys_str() const;
 
-    //! For AnyMaps which are derived from an input file, set the line and
-    //! column of this AnyMap in that file. Used for providing context for some
-    //! error messages.
-    void setLoc(int line, int column);
-
     //! Set a metadata value that applies to this AnyMap and its children.
     //! Mainly for internal use in reading or writing from files.
     void setMetadata(const std::string& key, const AnyValue& value);
 
-    //! Get a value from the metadata applicable to the AnyMap tree containing
-    //! this AnyMap.
-    const AnyValue& getMetadata(const std::string& key) const;
-
     //! Propagate metadata to any child elements
     void propagateMetadata(shared_ptr<AnyMap>& file);
 
@@ -516,23 +516,12 @@ class AnyMap
     //! The default units that are used to convert stored values
     UnitSystem m_units;
 
-    //! Starting line for this map in the input file
-    int m_line;
-
-    //! Starting column for this map in the input file
-    int m_column;
-
-    //! Metadata relevant to an entire AnyMap tree, such as information about
-    // the input file used to create it
-    shared_ptr<AnyMap> m_metadata;
-
     //! Cache for previously-parsed input (YAML) files. The key is the full path
     //! to the file, and the second element of the value is the last-modified
     //! time for the file, which is used to enable change detection.
     static std::unordered_map<std::string, std::pair<AnyMap, int>> s_cache;
 
     friend class AnyValue;
-    friend class InputFileError;
 };
 
 // Define begin() and end() to allow use with range-based for loops
@@ -553,7 +542,7 @@ class InputFileError : public CanteraError
     //! `node`. The `message` and `args` are processed as in the CanteraError
     //! class.
     template <typename... Args>
-    InputFileError(const std::string& procedure, const AnyValue& node,
+    InputFileError(const std::string& procedure, const AnyBase& node,
                    const std::string& message, const Args&... args)
         : CanteraError(
             procedure,
@@ -563,18 +552,21 @@ class InputFileError : public CanteraError
         }
 
     //! Indicate an error occurring in `procedure` while using information from
-    //! `node`. The `message` and `args` are processed as in the CanteraError
-    //! class.
+    //! `node1` and `node2`. The `message` and `args` are processed as in the
+    //! CanteraError class.
     template <typename... Args>
-    InputFileError(const std::string& procedure, const AnyMap& node,
-                   const std::string& message, const Args&... args)
+    InputFileError(const std::string& procedure, const AnyBase& node1,
+                   const AnyBase& node2, const std::string& message,
+                   const Args&... args)
         : CanteraError(
             procedure,
-            formatError(fmt::format(message, args...),
-                        node.m_line, node.m_column, node.m_metadata))
+            formatError2(fmt::format(message, args...),
+                         node1.m_line, node1.m_column, node1.m_metadata,
+                         node2.m_line, node2.m_column, node2.m_metadata))
         {
         }
 
+
     virtual std::string getClass() const {
         return "InputFileError";
     }
@@ -582,6 +574,9 @@ class InputFileError : public CanteraError
     static std::string formatError(const std::string& message,
                                    int line, int column,
                                    const shared_ptr<AnyMap>& metadata);
+    static std::string formatError2(const std::string& message,
+        int line1, int column1, const shared_ptr<AnyMap>& metadata1,
+        int line2, int column2, const shared_ptr<AnyMap>& metadata2);
 };
 
 }
diff --git a/interfaces/cython/cantera/ck2yaml.py b/interfaces/cython/cantera/ck2yaml.py
index 292264f320..5644539569 100644
--- a/interfaces/cython/cantera/ck2yaml.py
+++ b/interfaces/cython/cantera/ck2yaml.py
@@ -61,6 +61,14 @@
 
 BlockMap = yaml.comments.CommentedMap
 
+logger = logging.getLogger(__name__)
+loghandler = logging.StreamHandler(sys.stdout)
+logformatter = logging.Formatter('%(message)s')
+loghandler.setFormatter(logformatter)
+logger.handlers.clear()
+logger.addHandler(loghandler)
+logger.setLevel(logging.INFO)
+
 def FlowMap(*args, **kwargs):
     m = yaml.comments.CommentedMap(*args, **kwargs)
     m.fa.set_flow_style()
@@ -757,7 +765,7 @@ def warn(self, message):
         if self.warning_as_error:
             raise InputError(message)
         else:
-            logging.warning(message)
+            logger.warning(message)
 
     @staticmethod
     def parse_composition(elements, nElements, width):
@@ -1562,7 +1570,7 @@ def readline():
                             entry = []
                             if label not in self.species_dict:
                                 if skip_undeclared_species:
-                                    logging.info('Skipping unexpected species "{0}" while reading thermodynamics entry.'.format(label))
+                                    logger.info('Skipping unexpected species "{0}" while reading thermodynamics entry.'.format(label))
                                     continue
                                 else:
                                     # Add a new species entry
@@ -1612,7 +1620,7 @@ def readline():
                                 except Exception as e:
                                     error_line_number = self.line_number - len(current) + 1
                                     error_entry = ''.join(current).rstrip()
-                                    logging.info(
+                                    logger.info(
                                         'Error while reading thermo entry starting on line {0}:\n'
                                         '"""\n{1}\n"""'.format(error_line_number, error_entry)
                                     )
@@ -1620,7 +1628,9 @@ def readline():
 
                                 if label not in self.species_dict:
                                     if skip_undeclared_species:
-                                        logging.info('Skipping unexpected species "{0}" while reading thermodynamics entry.'.format(label))
+                                        logger.info(
+                                            'Skipping unexpected species "{0}" while'
+                                            ' reading thermodynamics entry.'.format(label))
                                         thermo = []
                                         line, comment = readline()
                                         current = []
@@ -1740,7 +1750,7 @@ def readline():
                             reaction, revReaction = self.read_kinetics_entry(kinetics, surface)
                         except Exception as e:
                             self.line_number = line_number
-                            logging.info('Error reading reaction starting on '
+                            logger.info('Error reading reaction starting on '
                                 'line {0}:\n"""\n{1}\n"""'.format(
                                     line_number, kinetics.rstrip()))
                             raise
@@ -1783,51 +1793,12 @@ def readline():
         for h in header:
             self.header_lines.append(h[indent:])
 
-        self.check_duplicate_reactions()
-
         for index, reaction in enumerate(self.reactions):
             reaction.index = index + 1
 
         if transportLines:
             self.parse_transport_data(transportLines, path, transport_start_line)
 
-    def check_duplicate_reactions(self):
-        """
-        Check for marked (and unmarked!) duplicate reactions. Raise exception
-        for unmarked duplicate reactions.
-
-        Pressure-independent and pressure-dependent reactions are treated as
-        different, so they don't need to be marked as duplicate.
-        """
-        possible_duplicates = defaultdict(list)
-        for r in self.reactions:
-            k = (tuple(r.reactants), tuple(r.products), r.kinetics.pressure_dependent)
-            possible_duplicates[k].append(r)
-
-        for reactions in possible_duplicates.values():
-            for r1,r2 in itertools.combinations(reactions, 2):
-                if r1.duplicate and r2.duplicate:
-                    pass  # marked duplicate reaction
-                elif (type(r1.kinetics) == ThreeBody and
-                      type(r2.kinetics) != ThreeBody):
-                    pass
-                elif (type(r1.kinetics) != ThreeBody and
-                      type(r2.kinetics) == ThreeBody):
-                    pass
-                elif (r1.third_body.upper() == 'M' and
-                      r1.kinetics.efficiencies.get(r2.third_body) == 0):
-                    pass  # explicit zero efficiency
-                elif (r2.third_body.upper() == 'M' and
-                      r2.kinetics.efficiencies.get(r1.third_body) == 0):
-                    pass  # explicit zero efficiency
-                elif r1.third_body != r2.third_body:
-                    pass  # distinct third bodies
-                else:
-                    raise InputError(
-                        'Encountered unmarked duplicate reaction {} '
-                        '(See lines {} and {} of the input file.).',
-                        r1, r1.line_number, r2.line_number)
-
     def parse_transport_data(self, lines, filename, line_offset):
         """
         Parse the Chemkin-format transport data in ``lines`` (a list of strings)
@@ -2009,9 +1980,9 @@ def convert_mech(input_file, thermo_file=None, transport_file=None,
 
         parser = Parser()
         if quiet:
-            logging.basicConfig(level=logging.ERROR)
+            logger.setLevel(level=logging.ERROR)
         else:
-            logging.basicConfig(level=logging.INFO)
+            logger.setLevel(level=logging.INFO)
 
         if permissive is not None:
             parser.warning_as_error = not permissive
@@ -2025,7 +1996,7 @@ def convert_mech(input_file, thermo_file=None, transport_file=None,
                 # Read input mechanism files
                 parser.load_chemkin_file(input_file)
             except Exception as err:
-                logging.warning("\nERROR: Unable to parse '{0}' near line {1}:\n{2}\n".format(
+                logger.warning("\nERROR: Unable to parse '{0}' near line {1}:\n{2}\n".format(
                                 input_file, parser.line_number, err))
                 raise
         else:
@@ -2040,8 +2011,8 @@ def convert_mech(input_file, thermo_file=None, transport_file=None,
                 parser.load_chemkin_file(thermo_file,
                                        skip_undeclared_species=bool(input_file))
             except Exception:
-                logging.warning("\nERROR: Unable to parse '{0}' near line {1}:\n".format(
-                                thermo_file, parser.line_number))
+                logger.warning("\nERROR: Unable to parse '{0}' near line {1}:\n".format(
+                               thermo_file, parser.line_number))
                 raise
 
         if transport_file:
@@ -2067,8 +2038,8 @@ def convert_mech(input_file, thermo_file=None, transport_file=None,
                 # Read input mechanism files
                 parser.load_chemkin_file(surface_file, surface=True)
             except Exception as err:
-                logging.warning("\nERROR: Unable to parse '{0}' near line {1}:\n{2}\n".format(
-                                surface_file, parser.line_number, err))
+                logger.warning("\nERROR: Unable to parse '{0}' near line {1}:\n{2}\n".format(
+                               surface_file, parser.line_number, err))
                 raise
 
         if extra_file:
@@ -2080,8 +2051,8 @@ def convert_mech(input_file, thermo_file=None, transport_file=None,
                 # Read input mechanism files
                 parser.load_extra_file(extra_file)
             except Exception as err:
-                logging.warning("\nERROR: Unable to parse '{0}':\n{1}\n".format(
-                                extra_file, err))
+                logger.warning("\nERROR: Unable to parse '{0}':\n{1}\n".format(
+                               extra_file, err))
                 raise
 
         if out_name:
@@ -2093,16 +2064,43 @@ def convert_mech(input_file, thermo_file=None, transport_file=None,
         surface_names = parser.write_yaml(name=phase_name, out_name=out_name)
         if not quiet:
             nReactions = len(parser.reactions) + sum(len(surf.reactions) for surf in parser.surfaces)
-            print('Wrote YAML mechanism file to {0!r}.'.format(out_name))
-            print('Mechanism contains {0} species and {1} reactions.'.format(len(parser.species_list), nReactions))
-        return surface_names
+            logger.info('Wrote YAML mechanism file to {0!r}.'.format(out_name))
+            logger.info('Mechanism contains {0} species and {1} reactions.'.format(
+                        len(parser.species_list), nReactions))
+        return parser, surface_names
+
+    def show_duplicate_reactions(self, error_message):
+        # Find the reaction numbers of the duplicate reactions by looking at
+        # the YAML file lines shown in the error message generated by
+        # Kinetics::checkDuplicates.
+        reactions = []
+        for line in error_message.split('\n'):
+            match = re.match('>.*# Reaction ([0-9]+)', line)
+            if match:
+                reactions.append(int(match.group(1))-1)
+
+        if len(reactions) != 2:
+            # Something went wrong while parsing the error message, so just
+            # display it as-is instead of trying to be clever.
+            logger.warning(error_message)
+            return
+
+        # Give an error message that references the line numbers in the
+        # original input file.
+        equation = str(self.reactions[reactions[0]])
+        lines = [self.reactions[i].line_number for i in reactions]
+        logger.warning('Undeclared duplicate reaction {}\nfound on lines {} and {} of '
+              'the kinetics input file.'.format(equation, lines[0], lines[1]))
 
 
 def convert_mech(input_file, thermo_file=None, transport_file=None,
                  surface_file=None, phase_name='gas', extra_file=None,
                  out_name=None, quiet=False, permissive=None):
-    return Parser.convert_mech(input_file, thermo_file, transport_file, surface_file,
-                               phase_name, extra_file, out_name, quiet, permissive)
+    _, surface_names = Parser.convert_mech(
+        input_file, thermo_file, transport_file, surface_file, phase_name,
+        extra_file, out_name, quiet, permissive)
+    return surface_names
+
 
 def main(argv):
 
@@ -2121,13 +2119,13 @@ def main(argv):
                                      repr(' '.join(args)))
 
     except getopt.GetoptError as e:
-        print('ck2yaml.py: Error parsing arguments:')
-        print(e)
-        print('Run "ck2yaml.py --help" to see usage help.')
+        logger.error('ck2yaml.py: Error parsing arguments:')
+        logger.error(e)
+        logger.error('Run "ck2yaml.py --help" to see usage help.')
         sys.exit(1)
 
     if not options or '-h' in options or '--help' in options:
-        print(__doc__)
+        logger.info(__doc__)
         sys.exit(0)
 
     input_file = options.get('--input')
@@ -2139,14 +2137,14 @@ def main(argv):
 
     if '--id' in options:
         phase_name = options.get('--id', 'gas')
-        logging.warning("\nFutureWarning: "
-                        "Option '--id=...' will be replaced by '--name=...'")
+        logger.warning("\nFutureWarning: "
+                       "Option '--id=...' will be replaced by '--name=...'")
     else:
         phase_name = options.get('--name', 'gas')
 
     if not input_file and not thermo_file:
-        print('At least one of the arguments "--input=..." or "--thermo=..."'
-              ' must be provided.\nRun "ck2yaml.py --help" to see usage help.')
+        logger.error('At least one of the arguments "--input=..." or "--thermo=..."'
+                     ' must be provided.\nRun "ck2yaml.py --help" to see usage help.')
         sys.exit(1)
 
     extra_file = options.get('--extra')
@@ -2160,9 +2158,9 @@ def main(argv):
     else:
         out_name = os.path.splitext(thermo_file)[0] + '.yaml'
 
-    surfaces = Parser.convert_mech(input_file, thermo_file, transport_file,
-                                   surface_file, phase_name, extra_file,
-                                   out_name, quiet, permissive)
+    parser, surfaces = Parser.convert_mech(input_file, thermo_file,
+            transport_file, surface_file, phase_name, extra_file, out_name,
+            quiet, permissive)
 
     # Do full validation by importing the resulting mechanism
     if not input_file:
@@ -2175,19 +2173,23 @@ def main(argv):
     try:
         import cantera as ct
     except ImportError:
-        print('WARNING: Unable to import Cantera Python module. Output '
-              'mechanism has not been validated')
+        logger.warning('WARNING: Unable to import Cantera Python module. '
+                        'Output mechanism has not been validated')
         sys.exit(0)
 
     try:
-        print('Validating mechanism...', end='')
+        logger.info('Validating mechanism...')
         gas = ct.Solution(out_name)
         for surf_name in surfaces:
             phase = ct.Interface(out_name, surf_name, [gas])
-        print('PASSED.')
+        logger.info('PASSED')
     except RuntimeError as e:
-        print('FAILED.')
-        print(e)
+        logger.info('FAILED')
+        msg = str(e)
+        if 'Undeclared duplicate reactions' in msg:
+            parser.show_duplicate_reactions(msg)
+        else:
+            logger.warning(e)
         sys.exit(1)
 
 
diff --git a/interfaces/cython/cantera/test/test_convert.py b/interfaces/cython/cantera/test/test_convert.py
index d94e11c93c..c31ef2cb0c 100644
--- a/interfaces/cython/cantera/test/test_convert.py
+++ b/interfaces/cython/cantera/test/test_convert.py
@@ -2,6 +2,9 @@
 from os.path import join as pjoin
 import itertools
 from pathlib import Path
+import logging
+import io
+import sys
 
 from . import utilities
 import cantera as ct
@@ -475,6 +478,36 @@ def test_extra(self):
         for key in ['foo', 'bar']:
             self.assertIn(key, yml.keys())
 
+    def test_duplicate_reactions(self):
+        # Running a test this way instead of using the convertMech function
+        # tests the behavior of the ck2yaml.main function and the mechanism
+        # validation step.
+
+        # clear global handler created by logging.basicConfig() in ck2cti
+        logging.getLogger().handlers.clear()
+
+        # Replace the ck2yaml logger with our own in order to capture the output
+        log_stream = io.StringIO()
+        logger = logging.getLogger('cantera.ck2yaml')
+        original_handler = logger.handlers.pop()
+        logformatter = logging.Formatter('%(message)s')
+        handler = logging.StreamHandler(log_stream)
+        handler.setFormatter(logformatter)
+        logger.addHandler(handler)
+
+        with self.assertRaises(SystemExit):
+            ck2yaml.main([
+                '--input={}/undeclared-duplicate-reactions.inp'.format(self.test_data_dir),
+                '--thermo={}/dummy-thermo.dat'.format(self.test_data_dir)])
+
+        # Put the original logger back in place
+        logger.handlers.clear()
+        logger.addHandler(original_handler)
+
+        message = log_stream.getvalue()
+        for token in ('FAILED', 'lines 12 and 14', 'R1A', 'R1B'):
+            self.assertIn(token, message)
+
 
 class CtmlConverterTest(utilities.CanteraTest):
     def test_sofc(self):
diff --git a/src/base/AnyMap.cpp b/src/base/AnyMap.cpp
index b070038ed3..8dee70a8ec 100644
--- a/src/base/AnyMap.cpp
+++ b/src/base/AnyMap.cpp
@@ -265,12 +265,32 @@ std::map<std::string, std::string> AnyValue::s_typenames = {
 
 std::unordered_map<std::string, std::pair<AnyMap, int>> AnyMap::s_cache;
 
+// Methods of class AnyBase
+
+AnyBase::AnyBase()
+    : m_line(-1)
+    , m_column(-1)
+{}
+
+void AnyBase::setLoc(int line, int column)
+{
+    m_line = line;
+    m_column = column;
+}
+
+const AnyValue& AnyBase::getMetadata(const std::string& key) const
+{
+    if (m_metadata && m_metadata->hasKey(key)) {
+        return m_metadata->at(key);
+    } else {
+        return Empty;
+    }
+}
+
 // Methods of class AnyValue
 
 AnyValue::AnyValue()
-  : m_line(-1)
-  , m_column(-1)
-  , m_key()
+  : m_key()
   , m_value(new boost::any{})
   , m_equals(eq_comparer<size_t>)
 {}
@@ -278,9 +298,7 @@ AnyValue::AnyValue()
 AnyValue::~AnyValue() = default;
 
 AnyValue::AnyValue(AnyValue const& other)
-    : m_line(other.m_line)
-    , m_column(other.m_column)
-    , m_metadata(other.m_metadata)
+    : AnyBase(other)
     , m_key(other.m_key)
     , m_value(new boost::any{*other.m_value})
     , m_equals(other.m_equals)
@@ -288,9 +306,7 @@ AnyValue::AnyValue(AnyValue const& other)
 }
 
 AnyValue::AnyValue(AnyValue&& other)
-    : m_line(other.m_line)
-    , m_column(other.m_column)
-    , m_metadata(std::move(other.m_metadata))
+    : AnyBase(std::move(other))
     , m_key(std::move(other.m_key))
     , m_value(std::move(other.m_value))
     , m_equals(std::move(other.m_equals))
@@ -298,11 +314,10 @@ AnyValue::AnyValue(AnyValue&& other)
 }
 
 AnyValue& AnyValue::operator=(AnyValue const& other) {
-    if (this == &other)
+    if (this == &other) {
         return *this;
-    m_line = other.m_line;
-    m_column = other.m_column;
-    m_metadata = other.m_metadata;
+    }
+    AnyBase::operator=(*this);
     m_key = other.m_key;
     m_value.reset(new boost::any{*other.m_value});
     m_equals = other.m_equals;
@@ -310,11 +325,10 @@ AnyValue& AnyValue::operator=(AnyValue const& other) {
 }
 
 AnyValue& AnyValue::operator=(AnyValue&& other) {
-    if (this == &other)
+    if (this == &other) {
         return *this;
-    m_line = other.m_line;
-    m_column = other.m_column;
-    m_metadata = std::move(other.m_metadata);
+    }
+    AnyBase::operator=(std::move(other));
     m_key = std::move(other.m_key);
     m_value = std::move(other.m_value);
     m_equals = std::move(other.m_equals);
@@ -351,21 +365,6 @@ const std::type_info &AnyValue::type() const {
     return m_value->type();
 }
 
-void AnyValue::setLoc(int line, int column)
-{
-    m_line = line;
-    m_column = column;
-}
-
-const AnyValue& AnyValue::getMetadata(const std::string& key) const
-{
-    if (m_metadata && m_metadata->hasKey(key)) {
-        return m_metadata->at(key);
-    } else {
-        return Empty;
-    }
-}
-
 void AnyValue::propagateMetadata(shared_ptr<AnyMap>& metadata)
 {
     m_metadata = metadata;
@@ -998,21 +997,6 @@ std::string AnyMap::keys_str() const
     return to_string(b);
 }
 
-void AnyMap::setLoc(int line, int column)
-{
-    m_line = line;
-    m_column = column;
-}
-
-const AnyValue& AnyMap::getMetadata(const std::string& key) const
-{
-    if (m_metadata && m_metadata->hasKey(key)) {
-        return m_metadata->at(key);
-    } else {
-        return Empty;
-    }
-}
-
 void AnyMap::propagateMetadata(shared_ptr<AnyMap>& metadata)
 {
     m_metadata = metadata;
@@ -1222,23 +1206,15 @@ AnyMap::Iterator end(const AnyValue& v) {
     return v.as<AnyMap>().end();
 }
 
-std::string InputFileError::formatError(const std::string& message,
-                                        int lineno, int column,
-                                        const shared_ptr<AnyMap>& metadata)
+namespace {
+void formatInputFile(fmt::memory_buffer& b, const shared_ptr<AnyMap>& metadata,
+        const std::string& filename, int lineno, int column, int lineno2=-1, int column2=-1)
 {
-    if (!metadata) {
-        return message;
+    if (lineno2 == -1) {
+        lineno2 = lineno;
+        column2 = column;
     }
-    std::string filename = metadata->getString("filename", "");
 
-    fmt::memory_buffer b;
-    format_to(b, "Error on line {} of", lineno+1);
-    if (filename.empty()) {
-        format_to(b, " input string:\n");
-    } else {
-        format_to(b, " {}:\n", filename);
-    }
-    format_to(b, "{}\n", message);
     format_to(b, "|  Line |\n");
     if (!metadata->hasKey("file-contents")) {
         std::ifstream infile(findInputFile(filename));
@@ -1248,16 +1224,68 @@ std::string InputFileError::formatError(const std::string& message,
     }
     std::string line;
     int i = 0;
+    int lastShown = -1;
     std::stringstream contents((*metadata)["file-contents"].asString());
     while (std::getline(contents, line)) {
-        if (lineno == i) {
+        if (i == lineno || i == lineno2) {
             format_to(b, "> {: 5d} > {}\n", i+1, line);
             format_to(b, "{:>{}}\n", "^", column + 11);
-        } else if (lineno + 4 > i && lineno < i + 6) {
+            lastShown = i;
+        } else if ((lineno + 4 > i && lineno < i + 6) ||
+                   (lineno2 + 4 > i && lineno2 < i + 6)) {
+            if (lastShown >= 0 && i - lastShown > 1) {
+                format_to(b, "...\n");
+            }
             format_to(b, "| {: 5d} | {}\n", i+1, line);
+            lastShown = i;
         }
         i++;
     }
+}
+}
+
+std::string InputFileError::formatError(const std::string& message,
+                                        int lineno, int column,
+                                        const shared_ptr<AnyMap>& metadata)
+{
+    if (!metadata) {
+        return message;
+    }
+    std::string filename = metadata->getString("filename", "input string");
+
+    fmt::memory_buffer b;
+    format_to(b, "Error on line {} of {}:\n{}\n", lineno+1, filename, message);
+    formatInputFile(b, metadata, filename, lineno, column);
+    return to_string(b);
+}
+
+std::string InputFileError::formatError2(const std::string& message,
+                                         int line1, int column1,
+                                         const shared_ptr<AnyMap>& metadata1,
+                                         int line2, int column2,
+                                         const shared_ptr<AnyMap>& metadata2)
+{
+    if (!metadata1 || !metadata2) {
+        return message;
+    }
+    std::string filename1 = metadata1->getString("filename", "input string");
+    std::string filename2 = metadata2->getString("filename", "input string");
+
+    fmt::memory_buffer b;
+    if (filename1 == filename2) {
+        format_to(b, "Error on lines {} and {} of {}:\n",
+                  std::min(line1, line2) + 1, std::max(line1, line2) + 1,
+                  filename1);
+        format_to(b, "{}\n", message);
+        formatInputFile(b, metadata1, filename1, line1, column1, line2, column2);
+    } else {
+        format_to(b, "Error on line {} of {} and line {} of {}:\n{}\n",
+                  line1+1, filename1, line2+1, filename2, message);
+        formatInputFile(b, metadata1, filename1, line1, column1);
+        format_to(b, "\n");
+        formatInputFile(b, metadata2, filename2, line2, column2);
+    }
+
     return to_string(b);
 }
 
diff --git a/src/kinetics/Kinetics.cpp b/src/kinetics/Kinetics.cpp
index caf77be85e..a9f04c7d12 100644
--- a/src/kinetics/Kinetics.cpp
+++ b/src/kinetics/Kinetics.cpp
@@ -156,7 +156,8 @@ std::pair<size_t, size_t> Kinetics::checkDuplicates(bool throw_err) const
                 }
             }
             if (throw_err) {
-                throw CanteraError("Kinetics::checkDuplicates",
+                throw InputFileError("Kinetics::checkDuplicates",
+                        R.input, other.input,
                         "Undeclared duplicate reactions detected:\n"
                         "Reaction {}: {}\nReaction {}: {}\n",
                         i+1, other.equation(), m+1, R.equation());
@@ -169,7 +170,8 @@ std::pair<size_t, size_t> Kinetics::checkDuplicates(bool throw_err) const
     if (unmatched_duplicates.size()) {
         size_t i = *unmatched_duplicates.begin();
         if (throw_err) {
-            throw CanteraError("Kinetics::checkDuplicates",
+            throw InputFileError("Kinetics::checkDuplicates",
+                m_reactions[i]->input,
                 "No duplicate found for declared duplicate reaction number {}"
                 " ({})", i, m_reactions[i]->equation());
         } else {
@@ -258,9 +260,10 @@ void Kinetics::checkReactionBalance(const Reaction& R)
         }
     }
     if (!ok) {
-        msg = "The following reaction is unbalanced: " + R.equation() + "\n" +
-              "  Element    Reactants    Products\n" + msg;
-        throw CanteraError("Kinetics::checkReactionBalance", msg);
+        throw InputFileError("Kinetics::checkReactionBalance", R.input,
+            "The following reaction is unbalanced: {}\n"
+            "  Element    Reactants    Products\n{}",
+            R.equation(), msg);
     }
 }
 
@@ -483,8 +486,8 @@ bool Kinetics::addReaction(shared_ptr<Reaction> r)
     // is not reversible, since computing the reverse rate from thermochemistry
     // only works for elementary reactions.
     if (r->reversible && !r->orders.empty()) {
-        throw CanteraError("Kinetics::addReaction", "Reaction orders may only "
-            "be given for irreversible reactions");
+        throw InputFileError("Kinetics::addReaction", r->input,
+            "Reaction orders may only be given for irreversible reactions");
     }
 
     // Check for undeclared species
@@ -493,9 +496,9 @@ bool Kinetics::addReaction(shared_ptr<Reaction> r)
             if (m_skipUndeclaredSpecies) {
                 return false;
             } else {
-                throw CanteraError("Kinetics::addReaction", "Reaction '" +
-                    r->equation() + "' contains the undeclared species '" +
-                    sp.first + "'");
+                throw InputFileError("Kinetics::addReaction", r->input,
+                    "Reaction '{}' contains the undeclared species '{}'",
+                    r->equation(), sp.first);
             }
         }
     }
@@ -504,9 +507,9 @@ bool Kinetics::addReaction(shared_ptr<Reaction> r)
             if (m_skipUndeclaredSpecies) {
                 return false;
             } else {
-                throw CanteraError("Kinetics::addReaction", "Reaction '" +
-                    r->equation() + "' contains the undeclared species '" +
-                    sp.first + "'");
+                throw InputFileError("Kinetics::addReaction", r->input,
+                    "Reaction '{}' contains the undeclared species '{}'",
+                    r->equation(), sp.first);
             }
         }
     }
@@ -515,8 +518,9 @@ bool Kinetics::addReaction(shared_ptr<Reaction> r)
             if (m_skipUndeclaredSpecies) {
                 return false;
             } else {
-                throw CanteraError("Kinetics::addReaction", "Reaction '{}' has "
-                    "a reaction order specified for the undeclared species '{}'",
+                throw InputFileError("Kinetics::addReaction", r->input,
+                    "Reaction '{}' has a reaction order specified for the "
+                    "undeclared species '{}'",
                     r->equation(), sp.first);
             }
         }
diff --git a/src/kinetics/KineticsFactory.cpp b/src/kinetics/KineticsFactory.cpp
index effed04a9d..998c9b10b5 100644
--- a/src/kinetics/KineticsFactory.cpp
+++ b/src/kinetics/KineticsFactory.cpp
@@ -177,6 +177,8 @@ void addReactions(Kinetics& kin, const AnyMap& phaseNode, const AnyMap& rootNode
             }
         }
     }
+
+    kin.checkDuplicates();
 }
 
 }
diff --git a/src/thermo/ThermoFactory.cpp b/src/thermo/ThermoFactory.cpp
index ff40c20ea0..1832095905 100644
--- a/src/thermo/ThermoFactory.cpp
+++ b/src/thermo/ThermoFactory.cpp
@@ -429,7 +429,7 @@ void addSpecies(ThermoPhase& thermo, const AnyValue& names, const AnyValue& spec
             if (species_nodes.count(name)) {
                 thermo.addSpecies(newSpecies(*species_nodes.at(name)));
             } else {
-                throw InputFileError("addSpecies", species,
+                throw InputFileError("addSpecies", names, species,
                     "Could not find a species named '{}'.", name);
             }
         }
diff --git a/test/data/undeclared-duplicate-reactions.inp b/test/data/undeclared-duplicate-reactions.inp
new file mode 100644
index 0000000000..80ffe18821
--- /dev/null
+++ b/test/data/undeclared-duplicate-reactions.inp
@@ -0,0 +1,15 @@
+ELEMENTS
+H  C
+END
+
+SPECIES
+H
+R1A R1B P1 R3 P3A P3B
+END
+
+REACTIONS
+R3+H <=> P3A+P3B             1.0e19    0.0     5000.0
+R1A+R1B <=> P1+H                1.0e19    0.0     5000.0
+R1A+R1B+M <=> P1+H+M            1.0e-2    0.0     5000.0
+P1+H => R1A+R1B                       1.0e19    0.0     5000.0
+END