The At-Least-Once Default
Most messaging systems promise to deliver each message at least once, not exactly once. The gap between what you assumed and what the system actually guarantees is where the duplicate-processing bugs live.
Correctness
The Idempotency Requirement
The moment you add retries to a system, you've made a promise you might not be keeping: that doing the same operation twice is the same as doing it once. Idempotency is what makes that promise true.