If there is a question whether a labor organization represents a majority of employees or whether the proposed unit is or is not appropriate, such matters shall be submitted to the State Conciliation Service for disposition. The State Conciliation Service shall promptly hold a public hearing and may, by decision, establish the boundaries of any collective bargaining unit and provide for an election to determine the question of representation. Provided, however, any certification of a labor organization to represent or act for the employees in any collective bargaining unit shall not be subject to challenge on the ground that a new substantial question of representation

within such collective bargaining unit exists until the lapse of one year from the date of certification or the expiration of any collective bargaining agreement, whichever is later.