re: nm

There does not appear to be anything in that statute which gives you protection or a legal basis for keeping the erroneous over-payment.

I'm sure your employer would have to show you that they in fact overpaid you (though that doesn't seem like it will be too difficult). Otherwise, they can likely recoup the payment that you A. did not earn and B. were not obligated to receive.

It might make you mad that they discovered your windfall, but I doubt you have any legal basis to keep the money.