An election shall not be directed in any bargaining unit or any subdivision within which, in the preceding 12 months, a valid election has been held. Employees engaged in an economic strike who are not entitled to reinstatement are eligible to vote, under regulations of the Board consistent with this Act, in any election conducted within 12 months after the beginning of the strike. In any election where none of the choices on the ballot receive a majority, a runoff shall be conducted by the Board. The ballot shall provide for a selection between the two choices receiving the largest and second largest number of valid votes cast in the election. (Added 1967, No. 198, § 9.)