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); "