Government Code § 31984

Any person who is eligible to participate in this system and who is a member of or participant in, or is eligible for membership or participation in, any other pension system provided by law for officers or employees of a county or judicial district may elect to become a member of this system, except as provided in Section 31982. Upon his election to participate in this system he is ineligible for membership or participation in any other retirement or pension system for officers or employees of a county or judicial district.