Why can a message from a queue arrive a second time?

The queue and the program doing the work don't always both know whether the job was completed. The program may have saved the result and then exited before acknowledging. The queue sees no acknowledgement and allows another attempt.

That's often a deliberate choice: better to allow a repeat than to lose a job. So handling has to be ready for duplicates. If a message says “add 10 to the counter”, without protection two deliveries add 20. You need a stable identifier of the same operation and a way to recognise that its effect has already been saved. The details of the guarantees depend on the queue used and its configuration.