GH#953 Add support for deduplicating headings

- Fixes PyCQA#953

isort/isort.py

@@ -86,6 +86,8 @@ def __init__(
             if key.startswith("import_heading") and value
         ]
 
+        self.seen_headings = set()
+
         self.line_separator = self.determine_line_separator(file_contents)
 
         self.in_lines = file_contents.split(self.line_separator)
@@ -753,11 +755,14 @@ def by_module(line: str) -> str:
                 )
                 if section_title:
                     section_comment = "# {}".format(section_title)
-                    if (
-                        section_comment not in self.out_lines[0:1]
-                        and section_comment not in self.in_lines[0:1]
-                    ):
-                        section_output.insert(0, section_comment)
+                    if section_title in self.seen_headings:
+                        if section_comment in self.out_lines[0:1]:
+                            self.out_lines = self.out_lines[1:]
+                    else:
+                        if section_comment not in self.out_lines[0:1]:
+                            section_output.insert(0, section_comment)
+                        if self.config['dedup_headings']:
+                            self.seen_headings.add(section_title)
 
                 if pending_lines_before or not no_lines_before:
                     output += [""] * self.config["lines_between_sections"]

isort/main.py

@@ -285,6 +285,12 @@ def parse_args(argv: Optional[Sequence[str]] = None) -> Dict[str, Any]:
         action="store_true",
         help="Only order imports alphabetically, do not attempt type ordering",
     )
+    parser.add_argument(
+        "--dont-dedup-headings",
+        action="store_false",
+        dest="dedup_headings",
+        help="Tells isort to repeat headings even if they have already been output.",
+    )
     parser.add_argument(
         "-e",
         "--balanced",
@@ -606,7 +612,7 @@ def parse_args(argv: Optional[Sequence[str]] = None) -> Dict[str, Any]:
     )
 
     arguments = {
-        key: value for key, value in vars(parser.parse_args(argv)).items() if value
+        key: value for key, value in vars(parser.parse_args(argv)).items() if value is not None
     }
     if "dont_order_by_type" in arguments:
         arguments["order_by_type"] = False

isort/settings.py

@@ -184,6 +184,7 @@ def _get_default(py_version: Optional[str]) -> Dict[str, Any]:
     "ignore_comments": False,
     "safety_excludes": True,
     "case_sensitive": False,
+    "dedup_headings": True,
 }