flask Returning Response and headers causes duplicate

The Fix

Upgrade to version 2.0.0 or later.

Based on closed pallets/flask issue #3628 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -27,9 +27,14 @@ Unreleased
 -   When specifying a factory function with ``FLASK_APP``, keyword
     argument can be passed. :issue:`3553`
--   When loading a ``.env`` or ``.flaskenv`` file on top level directory,
-    Flask will not change current work directory to the location of dotenv
-    files, in order to prevent potential confusion. :pr:`3560`

repro.py

from flask import Flask
app = Flask(__name__)
@app.route('/')
def issue():
    return {'test': 'test'}, {'Content-Type': 'test'}

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 2.0.0 or later.\nWhen NOT to use: This fix is not applicable if the application intentionally requires duplicate headers.\n\n

Why This Fix Works in Production

Trigger: Returning Response and headers causes duplicate headers
Mechanism: The issue arises from jsonify adding a 'Content-Type' header while make_response extends existing headers
Why the fix works: Changed `make_response` to use `headers.update` instead of `headers.extend`, resolving the issue of duplicate headers. (first fixed release: 2.0.0).

Production impact:

If left unfixed, retries/timeouts can trigger duplicate external side-effects (double charges, duplicate emails, repeated writes).

Why This Breaks in Prod

The issue arises from jsonify adding a 'Content-Type' header while make_response extends existing headers
Production symptom (often without a traceback): Returning Response and headers causes duplicate headers

Proof / Evidence

GitHub issue: #3628
Fix PR: https://github.com/pallets/flask/pull/3684
First fixed release: 2.0.0
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.63

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“I don't know if we should change this behavior, but if you want to set the default mimetype (e.g. Content-Type value) for JSON response, you…”

@greyli · 2020-05-30 · source

“I don't think he wants to change it globally. Since duplicate content-type headers make no sense I think it's reasonable to take the one from…”

@ThiefMaster · 2020-05-30 · source

“@greyli thanks, we need to support multiple different content-types for JSON in the same app. I think this is pretty common for production APIs that…”

@tonysimpson · 2020-06-01 · source

“Before we do anything, I want to understand why headers are extended rather than appended right now”

@davidism · 2020-06-01 · source

Failure Signature (Search String)

Returning Response and headers causes duplicate headers
Using `curl -v http://127.0.0.1:5000/` to query the view I expect only one `Content-Type` header > `Content-Type: test`

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Returning Response and headers causes duplicate headers
Using `curl -v http://127.0.0.1:5000/` to query the view I expect only one `Content-Type` header > `Content-Type: test`

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Returning Response and headers causes duplicate headers
Using `curl -v http://127.0.0.1:5000/` to query the view I expect only one `Content-Type` header > `Content-Type: test`

Minimal Reproduction

repro.py

from flask import Flask
app = Flask(__name__)
@app.route('/')
def issue():
    return {'test': 'test'}, {'Content-Type': 'test'}

What Broke

Duplicate headers are returned in HTTP responses, causing potential client confusion.

Why It Broke

The issue arises from jsonify adding a 'Content-Type' header while make_response extends existing headers

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Upgrade to version 2.0.0 or later.

When NOT to use: This fix is not applicable if the application intentionally requires duplicate headers.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/pallets/flask/pull/3684

First fixed release: 2.0.0

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

This fix is not applicable if the application intentionally requires duplicate headers.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
2.0.0	Fixed

Related Issues

No related fixes found.

Cluster: flask:duplicate-processing All hubs All clusters

Related clusters: Configuration error Data consistency Other

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

flask Returning Response and headers causes duplicate headers (Fix)