Idempotency
also idempotent request
A property where making the same request multiple times produces the same result as making it once, preventing duplicate side effects.
Analogy
Like pressing a floor button in an elevator twice: the second press changes nothing.
Why it matters
Idempotency keys stop retries and duplicate webhooks from creating double contacts or charging a customer twice.
In practice
Passing the same Idempotency-Key header so a retried payment isn't charged twice.