Celery A task will run multiple times when chaining chains

The Fix

pip install celery==5.5.0

Based on closed celery/celery issue #9020 · 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

@@ -974,9 +974,7 @@ def __or__(self, other):
         elif isinstance(other, _chain):
             # chain | chain -> chain
-            # use type(self) for _chain subclasses
-            return type(self)(seq_concat_seq(
-                self.unchain_tasks(), other.unchain_tasks()), app=self._app)

repro.py

app = Celery('tasks', broker='memory://', result_backend='cache+memory://')


@app.task(bind=True)
def task_a(self):
    print("task_a")


@app.task(bind=True)
def task_b(self):
    print('task_b')


@app.task(bind=True)
def task_c(self):
    print("task_c")


group1 = celery.group(task_a.si(), task_a.si())
group2 = celery.group(task_a.si(), task_a.si())
chord1 = group1 | group2
chord2 = celery.chain(chord1, (task_b.si() | task_c.si())).apply_async()

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\npip install celery==5.5.0\nWhen NOT to use: Do not use this fix if the task chaining logic is intentionally designed to allow multiple executions.\n\n

Why This Fix Works in Production

Trigger: - [x] I have included all related issues and possible duplicate issues
Mechanism: The logic for upgrading to a chord was not applied when chaining two chains, causing multiple task executions
Why the fix works: Fixes a bug where a task would run multiple times when chaining chains with groups. (first fixed release: 5.5.0).

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 logic for upgrading to a chord was not applied when chaining two chains, causing multiple task executions
Production symptom (often without a traceback): - [x] I have included all related issues and possible duplicate issues

Proof / Evidence

GitHub issue: #9020
Fix PR: https://github.com/celery/celery/pull/9021
First fixed release: 5.5.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.59

Discussion

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

Jump to Sources Open on GitHub

“ # Checklist  - [x] I hav”

Issue thread · issue description · source

Failure Signature (Search String)

- [x] I have included all related issues and possible duplicate issues
or possible duplicates to this issue as requested by the checklist above.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
- [x] I have included all related issues and possible duplicate issues
or possible duplicates to this issue as requested by the checklist above.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
- [x] I have included all related issues and possible duplicate issues
or possible duplicates to this issue as requested by the checklist above.

Minimal Reproduction

repro.py

app = Celery('tasks', broker='memory://', result_backend='cache+memory://')


@app.task(bind=True)
def task_a(self):
    print("task_a")


@app.task(bind=True)
def task_b(self):
    print('task_b')


@app.task(bind=True)
def task_c(self):
    print("task_c")


group1 = celery.group(task_a.si(), task_a.si())
group2 = celery.group(task_a.si(), task_a.si())
chord1 = group1 | group2
chord2 = celery.chain(chord1, (task_b.si() | task_c.si())).apply_async()

What Broke

Tasks were executed multiple times, leading to unexpected behavior and potential data inconsistencies.

Why It Broke

The logic for upgrading to a chord was not applied when chaining two chains, causing multiple task executions

Fix Options (Details)

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

pip install celery==5.5.0

When NOT to use: Do not use this fix if the task chaining logic is intentionally designed to allow multiple executions.

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

Fix reference: https://github.com/celery/celery/pull/9021

First fixed release: 5.5.0

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not use this fix if the task chaining logic is intentionally designed to allow multiple executions.

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
5.5.0	Fixed

Related Issues

No related fixes found.

Cluster: celery:duplicate-processing Celery hub Celery best practices All hubs All clusters

Related clusters: Configuration error Data consistency Duplicates

Sources

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

Celery A task will run multiple times when chaining chains with groups (Fix)