Wash. Rev. Code § 48.20.192
There may be a provision as follows:
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 be in force concurrently herewith, making the aggregate indemnity for . . . . . . . . . (insert type of coverage or coverages) in excess of $. . . . . . (insert maximum limit of indemnity or indemnities) the excess insurance shall be void and all premiums paid for such excess shall be returned to the insured or to his estate.
Or, in lieu thereof:
Insurance effective at any one time on the insured under a like policy or policies in this insurer is limited to the one such policy elected by the insured, his or her beneficiary or his estate, as the case may be, and the insurer will return all premiums paid for all other such policies.
RCW 48.20.192