Clean repeated entries in PATH after vcvars call

conans/client/tools/win.py

@@ -332,7 +332,18 @@ def vcvars_dict(settings, arch=None, compiler_version=None, force=False, filter_
                 # The new value ends with separator and the old value, is a list, get only the
                 # new elements
                 if old_value and value.endswith(os.pathsep + old_value):
-                    new_env[name_var] = value[:-(len(old_value) + 1)].split(os.pathsep)
+                    tmp = value[:-(len(old_value) + 1)].split(os.pathsep)
+                    if name_var.lower() == "path":
+                        # vcvars doesn't look if previous paths are already set so clean the repeated to avoid
+                        # max path (or any other list env var) len issues:
+                        new_env[name_var] = []
+                        uniques = []
+                        for i in tmp:
+                            if i and i.lower() not in uniques:
+                                uniques.append(i.lower())
+                                new_env[name_var].append(i)
+                    else:
+                        new_env[name_var] = tmp
                 elif value != old_value:
                     # Only if the vcvars changed something, we return the variable,
                     # otherwise is not vcvars related
@@ -355,7 +366,6 @@ def relevant_path(path):
 
     return new_env
 
-
 @contextmanager
 def vcvars(*args, **kwargs):
     if platform.system() == "Windows":

conans/test/util/tools_test.py

@@ -710,6 +710,38 @@ def vcvars_echo_test(self):
             self.assertIn("Conan:vcvars already set", str(output))
             self.assertIn("VS140COMNTOOLS=", str(output))
 
+    @unittest.skipUnless(platform.system() == "Windows", "Requires Windows")
+    def vcvars_env_not_duplicated_path_test(self):
+        """vcvars is not looking at the current values of the env vars, with PATH it is a problem because you
+        can already have set some of the vars and accumulate unnecessary entries."""
+        settings = Settings.loads(default_settings_yml)
+        settings.os = "Windows"
+        settings.compiler = "Visual Studio"
+        settings.compiler.version = "15"
+        settings.arch = "x86"
+        settings.arch_build = "x86_64"
+
+        # Set the env with a PATH containing the vcvars paths
+        tmp = tools.vcvars_dict(settings, only_diff=False)
+        tmp = {key.lower(): value for key, value in tmp.items()}
+        with tools.environment_append({"path": ";".join(tmp["path"])}):
+            previous_path = os.environ["PATH"].split(";")
+            # Duplicate the path, inside the tools.vcvars shouldn't have repeated entries in PATH
+            with tools.vcvars(settings):
+                path = os.environ["PATH"].split(";")
+                new_values = []
+                repeated_keys = []
+                for i in path:
+                    if i not in new_values:
+                        new_values.append(i)
+                    else:
+                        repeated_keys.append(i)
+
+        for repeated in repeated_keys:
+            if previous_path.count(repeated) < 2:
+                # If the entry was already repeated before calling "tools.vcvars" we keep it
+                raise AssertionError("The key '%s' was not repeated previously but now it is" % repeated)
+
     @unittest.skipUnless(platform.system() == "Windows", "Requires Windows")
     def vcvars_amd64_32_cross_building_support_test(self):
         # amd64_x86 crossbuilder