INSURANCE WITH OTHER INSURERS: If this insurer has not been given written notice before the occurrence or commencement of loss that the insured under this policy has other valid coverage, not with this insurer, and that other valid coverage provides benefits for the same loss on a provision of service basis or on an expense incurred basis, the only liability under any expense incurred coverage of this policy is for the proportion of the loss as the amount that would otherwise have been payable under this policy plus the total of the like amounts under all other valid coverages for the loss of which this insurer had notice bears to the total like amounts under all valid coverages for the loss, and for the return of the portion of the premium paid that exceeds the pro rata portion for the amount so determined. For the purpose of applying this provision when other coverage is on a provision of service basis, the term "like amount" means with respect to the other coverage the amount that the services rendered would have cost in the absence of the coverage.
MCL 500.3438