Reduce duplicate matches - ensures compatibility Responses 0.19.0 (#4914)

moto/core/responses_custom_registry.py

@@ -10,6 +10,10 @@ class CustomRegistry(responses.registries.FirstMatchRegistry):
      - CallbackResponses are not discarded after first use - users can mock the same URL as often as they like
     """
 
+    def add(self, response):
+        if response not in self.registered:
+            super().add(response)
+
     def find(self, request):
         all_possibles = responses._default_mock._registry.registered + self.registered
         found = []