Mont. Code § 33-22-224
"Other Insurance in This Insurer: If an accident or sickness or accident and sickness policy or policies previously issued by the insurer to the insured are in force concurrently with this policy, making the aggregate indemnity for .... (insert type of coverage or coverages) in excess of $.... (insert maximum limit of indemnity or indemnities), the excess insurance is void and all premiums paid for the excess must be returned to the insured or to the insured's estate."
"Insurance effective at any one time on the insured under a similar policy or policies in this insurer is limited to the one policy elected by the insured, the insured's beneficiary, or the insured's estate, as appropriate, and the insurer will return all premiums paid for all other policies."
§ 33-22-224, MCA