Design for at-least-once delivery

Cloudflare Queues provides at-least-once delivery by default. Reliability means a rare duplicate is possible even after normal success.

Give every operation a stable idempotency key. Enforce uniqueness in D1, a Durable Object, or the external provider. An in-memory Set disappears across isolates and deployments. Acknowledging before the side effect risks losing work.

Deliver the same payment-like message twice and prove the durable result is created once.

Use a stable message ID as the idempotency boundary:

insert into processed_messages (id, processed_at)
values (?, current_timestamp)
on conflict (id) do nothing;

Only perform the side effect when the insert claims the ID. Then deliberately retry the same message twice. A queue retry is normal delivery behavior, so duplicate handling belongs in the design rather than in an emergency cleanup script.