Iowa Code § 39.12: Failure to vacate.

An elected official who has been elected to another elective office to which section 39.11 applies shall choose only one office in which to serve unless otherwise permitted pursuant to section 39.11, subsection 2. The official shall resign from all but one of the offices to which section 39.11 applies before the beginning of the term of the office to which the person was most recently elected unless otherwise permitted pursuant to section 39.11, subsection 2. Failure to submit the required resignation will result in a vacancy in the first elective office to which the person was elected.