Fixed #11560 -- Allowed proxy model multiple-inheritance from the same concrete base model.

tests/proxy_models/models.py

@@ -110,10 +110,20 @@ class UserProxy(User):
         proxy = True
 
 
+class AnotherUserProxy(User):
+    class Meta:
+        proxy = True
+
+
 class UserProxyProxy(UserProxy):
     class Meta:
         proxy = True
 
+
+class MultiUserProxy(UserProxy, AnotherUserProxy):
+    class Meta:
+        proxy = True
+
 # We can still use `select_related()` to include related models in our querysets.