§ 309. Non-duplication. No compensation under the provisions of this article shall be payable to any civil defense volunteer or to the dependents of a deceased civil defense volunteer otherwise entitled to receive workers' compensation under the provisions of this chapter or of any other law, or volunteer firefighters' benefits or volunteer ambulance workers' benefits under the provisions of the volunteer firefighters' benefit law or the volunteer ambulance workers' benefit law or of any other law, or if the disabled civil defense volunteer or the dependents of a deceased civil defense volunteer are entitled to receive benefits under the disability benefits law or similar statute of any state or under any disability or retirement or insurance program of an employer who has contributed to the cost thereof or under any provision of benefits by or under laws of the federal government or a state or political subdivision thereof, except only as herein provided. Compensation payable under this article shall in such event, or any of them, be reduced to an amount which, together with such other compensation or benefits, shall not exceed in aggregate the amount of compensation to which the claimant, or his dependents, would be entitled under this article but for such other provision.