3.3.1 Idempotency
GET is naturally idempotent. POST often is not — a double-click can charge twice. An idempotency key lets the server return the first result again.
Why it matters
Retries and duplicate webhooks are normal. Without idempotency you create double orders.
How it is used
Client sends Idempotency-Key. Server stores the response by key for 24h. Unique constraints in the DB are the last line of defense.