Idempotency

Idempotency is a property of an action that means doing it several times has the same effect as doing it once. Pressing a lift call button ten times does not summon ten lifts; the extra presses change nothing. In software, an idempotent payment step can be safely retried without charging a customer twice.

It is like a light switch already in the on position: flicking it to on again leaves the room exactly as it was. Idempotency matters because networks drop and buttons get double-clicked, and actions built this way stay safe to repeat rather than causing costly duplicates.