Problem Definition
If a user A wants to make one payment to user B, and he does a REST request which blocks for some time, the user must not send a new request with a different payment identifier, otherwise it is possible that A will pay to B twice.
It's fine for the user A to send the same request twice, as long as the payment is still in flight. This however is susceptible to race conditions, so it's not recommended.
Problem Definition
If a user A wants to make one payment to user B, and he does a REST request which blocks for some time, the user must not send a new request with a different payment identifier, otherwise it is possible that A will pay to B twice.
It's fine for the user A to send the same request twice, as long as the payment is still in flight. This however is susceptible to race conditions, so it's not recommended.