The error message is sent in the API response when the Punchh server is getting two requests in the fraction of time difference. Now if Punchh receives one request on top of another with the same code which is still in process for the same user, the API response includes an error saying that "Another transaction with same code is processing, please try later".
The workaround for this error is that you wait for the response of the first redemption and then create a request for the second one.
If further assistance is required, submit a ticket to Punchh Support. (For help submitting a ticket, click here)