A customer paid us twice for the same invoice. How do we enter it and how do we pay the customer back?

I'm here to assist you in managing this overpayment correctly within QuickBooks Online (QBO), LGIPromo.

You have the option to record a refund to your customer using a check or expense. This will decrease your bank balance and offset the customer's outstanding credit, overpayment, or prepayment.

1. Go to the + New button. Then, select Expense.
2. Choose the customer you want to refund from the Payee drop-down list.
3. From the Payment Account  drop-down menu, select the bank account to which you deposited the overpayment to.
4. On the first line of the Category column, choose Accounts Receivable.
5. Enter how much you want to refund in the Amount field.
6. Fill out the other fields.
7. Hit Save and Close.

After completing the refund, we can associate it with the customer's credit or overpayment. Here's how:

1. Still, go to the + New and click Receive payment.
2. Choose the same customer you used for the check or expense when recording the refund.
3. Fill out the other fields as you see fit.
4. Under the Outstanding Transaction section, select the checkbox for the Expense or Check you created when recording the refund.
   
5. Make sure the payment is equal to the open balance, then click Save and Close. 

For more info, please check out this article: Record a customer refund in QuickBooks Online. 

Feel free to ask if you have any more questions about handling your customer's refund. I'm available to assist you at any time.