40 ILCS 5/20-120

In no event shall pension credit for the same service rendered by an employee be accredited in more than one participating system. If employment is covered by more than one participating system, pension credit shall be granted by that system which was first authorized to grant the credit, or if more than one participating system was authorized to grant credit at the same time, the employee shall elect, prior to retirement, the system under which credit shall be granted. The participating system under which pension credit is forfeited because of the application of this section, shall refund to the employee, the contributions for the period of service forfeited.