40 ILCS 5/20-119
Any employee who is concurrently employed by employers under 2 or more participating systems is entitled to establish pension credit in accordance with the provisions of each system.
If the concurrent employment results in duplication of credits, each of the systems shall reduce the service credit for the period of concurrent employment to its full-time equivalent, using as a basis for this adjustment, the earnings credited for each employment. However, no such reduction in service credit shall be applied for the purpose of meeting the one-year minimum service requirement in item (1) of Section 20-109, except as provided in Section 20-120.
Combined earnings credits shall be limited to the earnings credits which would have been established by full-time employment with the employer from which the employee was receiving the highest salary.
Seasonal employment covered by a retirement system during a period for which credit has been granted in another retirement system is concurrent employment within the meaning of this Section and no adjustment of the credits for seasonal employment is required, unless it results in a duplication of pension credits. If seasonal employment results in a duplication of credits, it shall be adjusted in accordance with Section 20-120.
40 ILCS 5/20-119