PayPal-Request-Id : Optional. Contains a unique user-generated ID that the server stores for a period of time. Use this header to enforce idempotency on REST API POST calls. You can make these calls any number of times without concern that the server creates or completes an action on a resource more than once. You can retry calls that fail with network timeouts or the HTTP 500 status code. You can retry calls for as long as the server stores the ID.
For example, a user calls refund captured payment with the PayPal-Request-Id header that contains a unique user-provided ID. The user can make the call again with the same ID in the PayPal-Request-Id header for up to 45 days because the server stores this ID for this long for this call.
If the initial call fails with the HTTP 500 status code but the server has already refunded the payment, the caller does not need to worry that the server will refund the payment again.