As a former Account Manager of 7 years dealing with payrioll and human resources (to name a few), I can tell you (like others already said) that you have no choice but to pay it back. Well, you have a choice obviously, but if you chose the wrong one they may take legal action. However, for roughly 5 bills I don't see a big company fighting for it as it would cost more upfront to recover that money (though the legal fees would eventually have to be reinbursed/paid for by, you guessed it, YOU).

Call your HR department and ask for a verification letter stating when you were overpaid AND ask for a copy of your payroll timecards. make sure to explain that you just want this information as back up and that you are more than willing to re-pay them. Like others have said, ask for a payment plan. They CAN do that. If they refuse, seek legal counseling or send whatever money you can, when you can. They wont have a choice but to accept it and it would show in a court of law that you were trying.