California Code, Family Code - FAM § 4007

(a) If a court orders a person to make specified payments for support of a child during the child's minority, or until the child is married or otherwise emancipated, or until the death of, or the occurrence of a specified event as to, a child for whom support is authorized under Section 3901 or 3910, the obligation of the person ordered to pay support terminates on the happening of the contingency.  The court may, in the original order for support, order the custodial parent or other person to whom payments are to be made to notify the person ordered to make the payments, or the person's attorney of record, of the happening of the contingency.

(b) If the custodial parent or other person having physical custody of the child, to whom payments are to be made, fails to notify the person ordered to make the payments, or the attorney of record of the person ordered to make the payments, of the happening of the contingency and continues to accept support payments, the person shall refund all moneys received that accrued after the happening of the contingency, except that the overpayments shall first be applied to any support payments that are then in default.