55 ILCS 5/1-5007
Any citizen and legal voter at general elections in said county may contest the right of any person, whose name is subscribed to said petition, to sign such petition under this Division, and shall also have the right to contest said petition as to any names subscribed thereto that he shall have good reason to believe, and does believe, as fictitious, and no other: Provided, he shall, 10 days before the first day of the said month of September file in the office of the clerk of the court of such county a list of the names of such persons whose right to sign such petition he is desirous of contesting, together with his affidavit, indorsed thereon, that he has good reason to believe, and does verily believe, that such persons named in said list are not legal voters of such county, and had no right in law to sign said petition; and shall also file in the office of said clerk, 10 days before said first day of September a list of such names as he has reason to believe are fictitious, together with his affidavit, indorsed thereon, that he has good reason to believe, and does verily believe, that such names are fictitious; and such persons shall have the right to contest such petitions only as to the names included in said lists.
55 ILCS 5/1-5007