This is very rare and is normally as a result of the App appearing "stuck" during card processing and the clerk terminating the app.
The result could be a missing transactions in the back-end because the app is not longer running OR duplicate transactions being made to the same card (in the merchant account) because the clerk thought the original transaction had not been processed.
Either way, there is no direct workaround for this other than:
- Re-constructing the transaction in the back-end based on what is seen in the merchant account
- Reversing the "duplicate" transaction in the merchant account (sales only)