It will arrive twice

The queue in this pipeline says “at least once” — the usual guarantee, and the one you almost always get. The worker calls the vendor, records the result, and deletes the job. What does that guarantee actually promise?

• That a job is delivered once, and lost only if the queue itself is lost.
• That a job is never lost — and nothing at all about it arriving only once.
• That duplicates are removed by the queue before the worker sees them.