The error was discovered through an investigation by ABC7's news partners The Daily Herald.
The Daily Herald says Duckworth claimed homestead exemptions in both DeKalb County and Cook County. By law, Illinois residents are allowed to claim exemption, but only on their primary residence.
Duckworth says she was unaware she had been claiming both exemptions simultaneously. She says she has paid more than $1,900 in back taxes, plus an additional $600 in fees.
Duckworth's Republican counterpart, Representative Joe Walsh, says he wants Duckworth to "release her tax returns from that period and apologize to voters of the 8th District."