Employer Overpayment

Hi, I'll try to keep this short. A former employer says they overpaid me. They would like their money back. I do not believe this is the case. Since this was their mistake that happened over a year ago, do I need to pay them? They are threatening a collection agency.
Thankyou

Was this alleged 'over-payment' on ONE paycheck or was over a lengthy period of time?

[racer72]

Hope you saved your paystubs. It does not matter if the overpayment was over a period of time or just a one time thing, if your former employer can prove it, you will have to pay the overpayment back.