An election shall 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. A rerun election may be conducted in the event of conduct improperly affecting a prior election. In an election involving more than 2 choices, where none of the choices on the ballot receives a majority vote, a runoff election shall be conducted between the 2 choices receiving the 2 largest numbers of valid votes cast in the election. An election shall not be directed in any bargaining unit or subdivision thereof where there is in force and effect a valid collective bargaining agreement which was not prematurely extended and which is of fixed duration. A collective bargaining agreement shall not bar an election upon the petition of persons not parties thereto where more than 3 years have elapsed since the agreement’s execution or last timely renewal, whichever was later.