store: redis
attempt 1: client gave up after 300 ms (the server is still working)
attempt 2, 300 ms later            -> HTTP 409  replayed=-    retry-after=1    {"title":"A request is outstanding for this Idempotency-Key","status":409}
attempt 3, after the first finished -> HTTP 201  replayed=true retry-after=-    {"id":1,"customer":"cus_42","amount":1999,"currency":"INR"}
payments in the table: 1