Why your integration tests pass but your message queue still double-processes in production

You know the story. You write a consumer, test it locally, deploy to staging, everything green. Then in production, the same message gets processed twice. Orders are duplicated. Notifications go out twice. Everyone blames "a race condition" and moves on. We spent the last few months formally verifying what actually happens when consumers crash with at-least-once delivery semantics. The short version: it doesn't matter which broker you use . The race condition is a property of the delivery contract, not a bu