Final answer:
If you see the message 'CREDIT AUTH EXISTS, ABORT PAYMENT,' you should select yes to prevent double charging the customer's credit card and ensure that the transaction is not processed twice.
Step-by-step explanation:
When a point of sale system displays the message CREDIT AUTH EXISTS, ABORT PAYMENT, it generally indicates that a credit authorization for the transaction has already been processed. In this situation, selecting yes to abort the payment would be the appropriate action. This prevents the customer's credit card from being charged twice for the same transaction. After aborting the payment, ensure that any confirmation of the initial credit authorization is provided to the customer, whether it be a receipt or a confirmation number.