(What a mouthful.)
This error typically occurs after a successful test transaction, just when you thought everything was going well!
In most cases, this means that the transaction ID submitted from your POS system to RedShelf has already been used once before. Perhaps a staff member edited the order and tried to re-process, but an easy way to validate is to log into the RedShelf account you used for the test transaction and check if the product was fulfilled.
If it was, then you can confidently ignore this message.
If it was not fulfilled, then please send a copy of any transaction details to firstname.lastname@example.org so we can investigate!