fix: Duplicate RateLimit-* headers with caching (#3855)

* create test to check headers values

* update headers values instead of adding duplicates

litestar/middleware/rate_limit.py

@@ -109,7 +109,7 @@ async def send_wrapper(message: Message) -> None:
                 message.setdefault("headers", [])
                 headers = MutableScopeHeaders(message)
                 for key, value in self.create_response_headers(cache_object=cache_object).items():
-                    headers.add(key, value)
+                    headers[key] = value
             await send(message)
 
         return send_wrapper

tests/unit/test_middleware/test_rate_limit_middleware.py

@@ -238,3 +238,22 @@ def handler() -> None:
         response = client.get("/src/static/test.css")
         assert response.status_code == HTTP_200_OK
         assert response.text == "styles content"
+
+
+async def test_rate_limiting_works_with_cache() -> None:
+    @get("/", cache=True)
+    def handler() -> None:
+        return None
+
+    config = RateLimitConfig(rate_limit=("minute", 2))
+    app = Litestar(route_handlers=[handler], middleware=[config.middleware])
+
+    with TestClient(app=app) as client:
+        response = client.get("/")
+        assert response.headers.get(config.rate_limit_remaining_header_key) == "1"
+
+        response = client.get("/")
+        assert response.headers.get(config.rate_limit_remaining_header_key) == "0"
+
+        response = client.get("/")
+        assert response.status_code == HTTP_429_TOO_MANY_REQUESTS