Idempotency

An operation is idempotent if running it more than once leaves the same result as running it once. Charging a card or sending a confirmation is dangerous to repeat, so designing those steps to recognize and ignore a repeat is what makes automatic retries safe.

It matters anywhere a message might be delivered twice or a job might restart mid-run. Without it, the very retry logic meant to make a system reliable can create duplicates. With it, 'try again' is always safe.