Re: Do i create an invoice for a payment that has already been received in quickbooks and shows as income in the bank register? will this create a duplicate income?
It depends how you recorded the payment.
1. If you actually recorded a "payment" then what it did was increase your bank account and increase accounts receivable for that customer. In that case, YES you should create the invocie and then apply the credit for that customer to the invoice. That way it hits your income account.
2. If you recorded the payment by entering a "bank deposit" then NO you don't need to create an invoice. You should have made sure the offsetting account on the deposit screen was whatever income account that payment represents.