409
idempotency_key_reusedplatformThis Idempotency-Key was used with a different request
Returned by the implementation; no specification rule declares it yet.
The key is known, and the body it arrived with does not match the body it arrived with the first time. paygate refuses rather than guessing which one you meant.
Why the API refuses
Idempotency-Key was used with a different request
What to do
Use one key per distinct request. The usual cause is a key derived from something too coarse — an order id reused for a second payment attempt with a different amount. Derive it from the operation, not from the entity. If you genuinely want to replay the original, send the original body and you will get the original response.