Under most circumstances I would return the check. I currently have one check deposited in my "Mad Money" account and it is for an invoice that was paid twice. A large national bank ordered and FHA appraisal and there was no problems/issues with the appraisal. The only problem was that it took them 11 months to pay the invoice. Their excuse, after almost weekly followups, was that they had already paid an invoice with my file # si I needed to assign it a new number. I did that and about 1 week later I received a check for the new file #. 3 days later I received a chack for the old file #. When they notify me of their mistake I will gladly refund them their money-after 11 months of gaining interest.
I agree with PE, what goes around, comes around. Remember USPAP was written to increase public confidence in our industry. If you cannot return money you were overpaid by, what does that say about the rest of your report and your character as a whole...so much for public confidence at that point, might as well just stop following USPAP as well.