Credit card processors and card providers will not allow two transactions for the same amount on the same card at the same credit card terminal.  It doesn't matter if the items are completely different, they only look at the price.  There are several things that you can do in a situation like this.

  1. Have them use a different card.
  2. Put the transaction on Hold and ring the transaction up at a different register.
  3. They can buy something else so the total is different.
  4. You can close the batch on the terminal (you don't need to end the shift) and then ring the transaction up again.