Fixed #25246 -- Guarded against duplicate paths in AppConfig.

django/apps/config.py

@@ -62,6 +62,10 @@ class AppConfig(object):
             filename = getattr(module, '__file__', None)
             if filename is not None:
                 paths = [os.path.dirname(filename)]
+            else:
+                # For unknown reasons, sometimes the list returned by __path__
+                # contains duplicates that must be removed (#25246).
+                paths = list(set(paths))
         if len(paths) > 1:
             raise ImproperlyConfigured(
                 "The app module %r has multiple filesystem locations (%r); "

tests/apps/tests.py

@@ -365,6 +365,14 @@ class AppConfigTests(SimpleTestCase):
         with self.assertRaises(ImproperlyConfigured):
             AppConfig('label', Stub(__path__=['a', 'b']))
 
+    def test_duplicate_dunder_path_no_dunder_file(self):
+        """
+        If the __path__ attr contains duplicate paths and there is no
+        __file__, they duplicates should be deduplicated (#25246).
+        """
+        ac = AppConfig('label', Stub(__path__=['a', 'a']))
+        self.assertEqual(ac.path, 'a')
+
 
 @skipUnless(six.PY3, "Namespace packages sans __init__.py were added in Python 3.3")
 class NamespacePackageAppTests(SimpleTestCase):