A property of operations where performing them multiple times produces the same result as performing them once.
In carrier integrations, idempotency prevents duplicate shipments when retrying failed requests. HTTP methods GET, PUT, and DELETE are naturally idempotent. POST is not — you need an Idempotency-Key header or client-side deduplication.
Implementation: generate a UUID for each logical operation, send it as a header, and store it server-side. If the carrier sees the same key twice, it returns the original response instead of creating a duplicate.