Hi Valy,
Sounds like you're on the right track by creating a credit to handle the extra payments. To process a refund for the double payment and ensure the customer receives the money back in their bank account, you'll need to go to the "Returns & Credit" tab, find the credit note, and hit "Pay Refund." After processing the refund, go to Banking > Prepare Electronic Payments and select the refund transaction to create a bank file. For more info, please check out this Help article: Customer Overpayments.
Feel free to get in touch if you need more help.
Cheers,
Princess
