An election must not be directed in any bargaining unit or any subdivision within which, in the preceding 12-month period, a valid election was held. The commission shall determine who is eligible to vote in the election and shall promulgate rules governing the election. In an election involving more than 2 choices, if none of the choices on the ballot receives a majority vote, a runoff election must be conducted between the 2 choices receiving the 2 largest numbers of valid votes cast in the election. An election must not be directed in any bargaining unit or subdivision of any bargaining unit if there is in force and effect a valid collective bargaining agreement that was not prematurely extended and that is of fixed duration. A collective bargaining agreement does not bar an election upon the petition of persons not parties to the collective bargaining agreement if more than 3 years have elapsed since the agreement's execution or last timely renewal, whichever was later.