If you receive a duplicate payment from a customer (a second payment on the same invoice) call the customer to tell them of their error. Do it today and do not hesitate to get on the telephone until you reach someone. Your customer may or may not catch this error depending upon their accounting system, but regardless, call them to let them know immediately that they paid you too much on an invoice.
Give your customer the option to choose what you will do with their duplicate payment. Ask them if they would like the money to be applied to other invoices. Do they want the check returned? Give them an option on your telephone call after you politely notify them of their accounting error. You want more business from them in the future so treat them honestly and openly. Your good reaction will remind them to call you for more business. Who does not want to deal with openly honest people?
Do not hold their payment hostage. Do not assume they have enough cash to allow you to keep this payment. You might ask them politely if you can apply it to another invoice but give them the choice.
Ask the customer what they want you to do. Remember that the money you received in error may be needed for payroll this week. Call them and ask; give them the opportunity to make that choice.
Do they want the check voided? Ask them if they want you to void the check before you send it back. If you have other outstanding invoices, they may be willing to allow you to keep the check as a remittance for other invoices. If that is the case, ask them to prepare a new customer remittance (statement of invoices paid) and forward it to you.
No matter the financial situation, call your customer immediately; you have no choice. Regardless of this outcome, call the customer and tell them they have paid the same invoice(s) twice. Be honest. Tell them you do not want two payments; you just want to be paid once and on time. They will appreciate this honest direct approach. The right reaction may impact the future when they decide to hire a company like yours.
Call them quickly so you can prove that you are truly honest. If your customer has to find out about this duplicate payment after several days or weeks and you have not called him to tell him of his error, he will never believe you again. It makes no difference if the poor judgment stemmed from your accounts payable person, everyone at the customer’s facility involved with your account will know you took the money and said nothing. How do you think they should regard you now? Think about the same situation if you paid one of your vendors too much and they said nothing? How much longer would you deal with this firm?