Did one process as primary and the other as secondary? What types of plans are they? These things matter in determining if there is an overpayment and who is entitled to it. For example, if the patient purchased insurance such as AFLAC, etc, they may be entitled to the difference. If both plans are employer group plans and both processed as primary, then there is a COB issue that needs to be resolved. Can't tell without knowing more about the two plans.