What do I do if host timeout occurred and customer was charged?

During an in store pos debit transaction with cash back, a host timeout occurred. Customer was charged amount of sale plus cash back requested and at same time just seconds later another charge was attempted, but just for sale amount and that was declined because customer only had enough for the first charge. On my end it just shows the one transaction that was declined but on customer statement it shows 2 transactions. Customer disputed with bank and was denied saying no error occurred, but it is obvious an error did indeed occurs. What can I do to fix this?