40 ILCS 5/20-118
Any employee who shall have waived, by the acceptance of a refund, his pension credit in any participating system, may have his pension credit reinstated by repayment of the refund, including interest from the date of refund to the date of repayment, provided (1) the system is authorized by law to receive the repayment, and (2) the employee has completed at least 2 years of service under a participating system subsequent to the date of the last refund. Each system shall consider pension credits under all participating systems in determining whether the employee meets the service requirements under that system for repayment of a refund. The repayment of a refund under this section shall not be considered as an election to accept the provisions of the other sections of this Article.
40 ILCS 5/20-118