409
idempotency_in_progressplatformA request with this key is still being processed
Returned by the implementation; no specification rule declares it yet.
The first request with this key has not finished. Answering would mean either running the operation twice or inventing a result, so paygate answers 409 and lets you ask again.
Why the API refuses
a request with this Idempotency-Key is still being processed
What to do
Retry the same request, with the same key, after a short delay — a few hundred milliseconds, then back off. Do not switch to a new key: that starts a second real operation, which is exactly what the key was there to prevent.