deduplicate content-type headers

CHANGES

@@ -5,6 +5,7 @@
   through which function they would like to send real requests
 * Added support for re.Pattern based header matching.
 * Added support for gzipped response bodies to `json_params_matcher`.
+* Fix `Content-Type` headers issue when the header was duplicated. See #644
 * Moved types-pyyaml dependency to `tests_requires`
 
 0.23.3

responses/__init__.py

@@ -491,10 +491,17 @@ def _req_attr_matches(
 
     def get_headers(self) -> HTTPHeaderDict:
         headers = HTTPHeaderDict()  # Duplicate headers are legal
-        if self.content_type is not None:
+
+        # Add Content-Type if it exists and is not already in headers
+        if self.content_type and (
+            not self.headers or "Content-Type" not in self.headers
+        ):
             headers["Content-Type"] = self.content_type
+
+        # Extend headers if they exist
         if self.headers:
             headers.extend(self.headers)
+
         return headers
 
     def get_response(self, request: "PreparedRequest") -> HTTPResponse:

responses/tests/test_responses.py

@@ -1330,6 +1330,29 @@ def run():
     assert_reset()
 
 
+def test_headers_deduplicated_content_type():
+    """Test to ensure that we do not have two values for `content-type`.
+
+    For more details see https://github.com/getsentry/responses/issues/644
+    """
+
+    @responses.activate
+    def run():
+        responses.get(
+            "https://example.org/",
+            json={},
+            headers={"Content-Type": "application/json"},
+        )
+        responses.start()
+
+        resp = requests.get("https://example.org/")
+
+        assert resp.headers["Content-Type"] == "application/json"
+
+    run()
+    assert_reset()
+
+
 def test_content_length_error(monkeypatch):
     """
     Currently 'requests' does not enforce content length validation,