If you have received a duplicate business or individual income tax refund, cash only one check. Cashing both checks may result in a fee from your financial institution. You can either destroy the second check or mail it back to Illinois State Comptroller at 325 West Adams, Springfield, Illinois 62704. Do not mail your check to the Illinois Department of Revenue. If you have any further questions, please contact the department at 800 732-8866 or 217 782-3336.