Ooohhhh...okay, well, you definitely have to refund the money (ERISA or not); you have a legal responsibility to immediately refund overpayments caused by incorrect coding, once the problem is identified. The problem, here, is that it's never a good idea to send an unsolicited refund to Aetna (as I have been warned against doing, by Aetna)...If the refund doesn't accompany an Accent overpayment account, it just sends them into a tizzy, and they have absolutely no idea what to do with it. We even had them hold up one of our paid EOB's for 6 months in "reprocessing," because there was a refund mix-up caused by an unsolicited refund on one account.
The moral of the story is, if you have to send a refund to Aetna, you should contact them and find out if they can send you a refund request if you identify an overpayment; don't just send it in.
Also, don't wait to contact them about an overpayment, hoping they won't notice and will leave you alone. It's not your doctor's money; they don't get to keep it. Holding on to it when you know you've got an error is playing Russian Roulette with the OIG.
Since you're already getting in refund requests, I would advise you to put together a spreadsheet of all of the overpayments you've identified (run a report if you have to), with every account, DOS, and payment amount that has to be returned. Then contact an Accent representative (Or whatever 3rd party company is handling your refund requests) and tell them that you need assistance in returning overpayments to Aetna. You need to compare which accounts they have sent refund requests on, to the accounts that you have, so that you can see what all has been refunded, and can send the outstanding overpayments in a way that they will be correctly accounted for and applied. It's going to be a lot of work, but it's the least you can do to remain compliant.