35 ILCS 5/906

If a protest has been filed with respect to a notice of deficiency issued by the Department with respect to a taxable year, and the decision of the Department on such protest has become final, the Department shall be barred from issuing a further or additional notice of deficiency for such taxable year, except in the case of fraud, mathematical error, a return that is not considered processable, as the term is defined in Section 3-2 of the Uniform Penalty and Interest Act, or as provided in section 905(d), (e), or (g).