(1)(a) The commission, upon reasonable notice, shall decide in each application for certification as an exclusive bargaining representative, the unit appropriate for the purpose of collective bargaining.

(b) In determining, modifying, or combining the bargaining unit, the commission shall consider the duties, skills, and working conditions of the employees; the history of collective bargaining by the employees and their bargaining representatives; the extent of organization among the employees; and the desire of the employees.

(2) The commission shall determine the bargaining representative by conducting an election after a showing of interest by employees, as provided in RCW 49.37.050.

(3) If a single employee organization is the exclusive bargaining representative for two or more units, upon petition by the employee organization, the units may be consolidated into a single larger unit if the commission considers the larger unit to be appropriate. If consolidation is appropriate, the commission shall certify the employee organization as the exclusive bargaining representative of the new unit.

(4) No question concerning representation may be raised if:

(a) Fewer than 12 months have elapsed since the last certification or election; or

(b) A valid collective bargaining agreement is in effect, except for that period of no more than 90 calendar days nor less than 60 calendar days before the expiration of the agreement.