If you were truly overpaid--that is, received more than you were supposed to--then you would have to repay the excess. If you don't believe this was the case, you could fight the request for overpayment, but would almost certainly need an attorney to do so--this area of law is complex. If the attorney cost would exceed the cost of repayment, it may not be worthwhile fighting, even if you believe you are in the right.