Idempotent

An operation is idempotent when applying it multiple times has the same effect as applying it once. Setting a value to 5 is idempotent; incrementing a value by 1 is not. The first call may change state, but every subsequent call with the same input leaves the state unchanged.

Idempotency matters for retries and at-least-once delivery: if a request can be safely repeated, a client can retry on timeout without risking duplicate effects.