My clients are starting to receive notices that they got an overpayment at the end of the year. Apparently, there was a glitch in OWCP's system and many people received a duplicate payment at the end of the year. You may also want to call in to OWCP to verify the correct debt number, for some reason many letters list an incorrect debt number, compounding this mess.
If you return the money online using pay.gov, I suggest that you upload a copy of the receipt you receive to your file so that the person handling the overpayment sees that payment. And if you mail in the payment, make sure your file number and the correct debt number are marked on the memo portion of the check, and I also recommend that you upload a copy of what you mailed and a copy of the check to your file. Its my experience that when sending in these repayments that it can take a long time for OWCP to receive notification that your payment was received, so the claims examiner they may not realize you have made that payment and escalate the overpayment to the next step.