From fd968a3ffaf2f6f1f0a7c07d2952e8d02c65da4b Mon Sep 17 00:00:00 2001
From: Maksim Beliaev <beliaev.m.s@gmail.com>
Date: Sat, 23 Sep 2023 01:00:50 +0200
Subject: [PATCH] deduplicate content-type headers

---
 CHANGES                           |  1 +
 responses/__init__.py             |  9 ++++++++-
 responses/tests/test_responses.py | 23 +++++++++++++++++++++++
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/CHANGES b/CHANGES
index 59a59252..a1d27a3a 100644
--- a/CHANGES
+++ b/CHANGES
@@ -3,6 +3,7 @@
 
 * 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
diff --git a/responses/__init__.py b/responses/__init__.py
index fb0a5fc2..655c99cd 100644
--- a/responses/__init__.py
+++ b/responses/__init__.py
@@ -479,10 +479,17 @@ 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:
diff --git a/responses/tests/test_responses.py b/responses/tests/test_responses.py
index 2ae3d655..02db63e2 100644
--- a/responses/tests/test_responses.py
+++ b/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,