If you were unaware of the overpayment and did not contribute to it--that is, you reported all you mileage correctly, and to the best of your knowledge, as determined by what was written on  any check stubs or other documentation you received, you were paid correctly, then you would not have to return the money you are not liable for the company's internal mistake when you played no role in it and did not know you were being overpaid.However, if when you received the checks, you became aware you were being overpaid e.g. you saw you were being paid more than you believed you should have been, then you'd have to repay the surplus. In this case, you are not blameless you had knowledge that you were not entitled to some of the money, but despite knowing that, kept the funds.