(a) Service rendered by an employee as a police officer and member of the regularly constituted police department of the city, or as a firefighter and regular member of the paid fire department of the city, or as a teacher in the public school system in the city shall be counted, for the purposes of this Article, as service rendered as an employee of the city. Salary received for any such service shall be treated, for the purposes of this Article, as salary received for the performance of duty as an employee.
     (b) Subsection (a) applies to service rendered after the effective date only if the employee pays to the Fund, prior to separation from service, an amount equal to what would have accumulated in his or her account from salary deductions as employee contributions, including interest at the effective rate, if such contributions had been made for age and service and spouse’s annuity during all of such service; provided, that no service shall be counted or payments received for any period of service for which the employee retains or has not forfeited his or her rights to credit for the same period of service in another annuity and benefit fund, or pension fund, in operation in the city for the benefit of such police officers, firefighters, or teachers. The amount transferred to the Fund under item (1) of Section 5-233.1, if any, shall be credited against the contributions required under this subsection.