double counting bug in gemini yield

Author: amiyapatanaik

pydantic_ai_slim/pydantic_ai/models/gemini.py

@@ -434,13 +434,13 @@ async def _get_gemini_responses(self) -> AsyncIterator[_GeminiResponse]:
             responses_to_yield = gemini_responses[:-1]
             for r in responses_to_yield[current_gemini_response_index:]:
                 current_gemini_response_index += 1
-                self._usage += _metadata_as_usage(r)
+                self._usage = _metadata_as_usage(r)
                 yield r
 
         # Now yield the final response, which should be complete
         if gemini_responses:
             r = gemini_responses[-1]
-            self._usage += _metadata_as_usage(r)
+            self._usage = _metadata_as_usage(r)
             yield r
 
     @property