Section 8. Appointing authorities and employees serving in positions subject to the provisions of chapter thirty-one on the effective date of this chapter and their successors, shall have the option to appeal to either the civil service commission or the local merit appeals board, but not both, in the following instances:
(a) a disciplinary action,
(b) a finding that the examination fairly tested the knowledge, skills and abilities required to perform the primary or dominant duties of the position for which the examination was held, or
(c) the results of a performance evaluation.
An employee covered by a collective bargaining agreement may continue to exercise any grievance arbitration procedure in said agreement and upon the election of such option by the employee, it shall be the exclusive remedy for resolving such matters. In any disciplinary action or discharge where the substance of such action was based on a performance evaluation the validity of which has been previously determined, the substantive matters involved in the evaluation shall not be open to redetermination by the commission or the board.