[4.1.x] Fixed #33800 -- Fixed system check for the same template tag module in installed apps and template tag libraries.

Thanks Claude Paroz for the report.

Regression in 004b4620f6.
Backport of 083bfca6b6 from main

django/core/checks/templates.py

@@ -50,15 +50,15 @@ def check_string_if_invalid_is_string(app_configs, **kwargs):
 @register(Tags.templates)
 def check_for_template_tags_with_the_same_name(app_configs, **kwargs):
     errors = []
-    libraries = defaultdict(list)
+    libraries = defaultdict(set)
 
     for conf in settings.TEMPLATES:
         custom_libraries = conf.get("OPTIONS", {}).get("libraries", {})
         for module_name, module_path in custom_libraries.items():
-            libraries[module_name].append(module_path)
+            libraries[module_name].add(module_path)
 
     for module_name, module_path in get_template_tag_modules():
-        libraries[module_name].append(module_path)
+        libraries[module_name].add(module_path)
 
     for library_name, items in libraries.items():
         if len(items) > 1:
@@ -66,7 +66,7 @@ def check_for_template_tags_with_the_same_name(app_configs, **kwargs):
                 Error(
                     E003.msg.format(
                         repr(library_name),
-                        ", ".join(repr(item) for item in items),
+                        ", ".join(repr(item) for item in sorted(items)),
                     ),
                     id=E003.id,
                 )

tests/check_framework/test_templates.py

@@ -158,6 +158,19 @@ class CheckTemplateTagLibrariesWithSameName(SimpleTestCase):
                 [self.error_same_tags],
             )
 
+    @override_settings(
+        INSTALLED_APPS=["check_framework.template_test_apps.same_tags_app_1"]
+    )
+    def test_template_tags_same_library_in_installed_apps_libraries(self):
+        with self.settings(
+            TEMPLATES=[
+                self.get_settings(
+                    "same_tags", "same_tags_app_1.templatetags.same_tags"
+                ),
+            ]
+        ):
+            self.assertEqual(check_for_template_tags_with_the_same_name(None), [])
+
     @override_settings(
         INSTALLED_APPS=["check_framework.template_test_apps.same_tags_app_1"]
     )