The thing to do, as you will see that I always advise, if you read any of the previous posts, is to respond to them, agree if it happens to be the truth (and DO NOT try to claim it's not if it is, as they have records of what you made. Good ones.) And indicate your willingness to correct the overpayment if one exists. If there is a genuine mistake, or if your employer's records are wrong, they'll help you figure it out.
But the internet is just full of people who are dying to get on and tell you that yes, you're going to jail, yes, you're going to have your home and children taken from you, yes, you need to get a lawyer and don't admit to anything. But that's very bad advice. I used to deal with unemployment collections, and your situation is common as dirt, and the agency which deals with you is much more interested in getting this overpayment corrected and getting their money back than they are in prosecuting you criminally. So get with them, and work it out. Don't waste the money you'd spend paying back the overpayment getting an attorney and trying to "fight" them. if you did it, they can tell you did it, as I said, they keep very good records you probably didn't know about, so being coy doesn't work to your advantage.