In 1698, Duncombe was arrested and imprisoned in the Tower of London, and subsequently expelled from the House of Commons, for "contriving and advising the making of false Indorsements of several Bills, made forth at the Receipt of Exchequer, commonly called Exchequer-Bills", in other words a tax fraud. He was expelled from the House of Commons on 1 February 1698.
Duncombe's misdeeds led to his detention at the Tower of London in 1698 and subsequently his exclusion from House of Commons proceedings, as a result of his involvement in forging false Exchequer-Bills, a tax fraud. His expulsion occurred on February 1, 1698.