If you're charged twice on a debit card, but the retailer immediately voids one of the transactions, then you might not need to do anything. If you don't need the money, and you can wait for the transaction to be fully voided, then you can simply wait. If you need the money soon, then you should ask the retailer to return the funds, rather than simply voiding the transaction. If neither you nor the retailer catches the double charge in time, then you may have to dispute the transaction with your bank.