Willful Acceptance of Overpaid Child Support Beyond 5 years!

Custodial Parent (CP) has knowingly accepted child support overpayments from Non-Custodial Parent (NCP). NCP attempted to locate CP and minor child for a number of years; only recently did NCP discover their whereabouts and child support has continue for child nearly 24 years old. This is a non-public assistance case with no arrearages since child support payment order. NCP has requested overpaid money back to no avail and wishes to pursue legal action; how to proceed?
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