Apology offered... I jumped right to the ethical instead of the accounting. I work in a payroll office and you'd be surprised how many people don't want to pay back errors. (Although they are the first to come crying when their check is five bucks smaller than normal because of something like taxes!)
I can't answer specifically for your state or even your office, but where I work we would only ask you for the *net* overpayment. (Meaning, we would take the gross, add applicable taxable benefits, subtract taxes and taxable deductions... I don't have the "formula" in front of me, but that's the gist of it.) All appropriate employer/employee contributions are adjusted to reflect the accurate final payment.
It gets even easier if the employee hasn't cashed the check yet, because then we just reverse the check, which reverses all of the contributions, deductions, etc. Then a new accurate check is issued with the accurate deductions and contributions.
But it depends on how your payroll system/office/state works.
I hope this makes sense... and hope it helps.