An operation is idempotent when repeating it with the same intended input has the same effect as performing it once.

Setting a resource to a particular value can be idempotent; incrementing it is not.

Idempotency makes retries safer because duplicate execution does not compound the state change. When the operation itself is not naturally idempotent, a protocol can attach a unique request identifier and remember completed identifiers so repeated delivery returns the previous result instead of performing the effect again.

This deduplication needs a defined scope and retention period. Forgetting an identifier too early can allow an old retry to execute again.

Idempotency is an application property. A reliable transport can retransmit bytes, but it cannot decide whether executing the represented business operation twice is acceptable.