He was educated at public schools in the Lismore area and became a dairy farmer at Konorigan.
[1] In 1965, Duncan was preselected as one of two Country Party candidates for the marginal Labor seat of Lismore, held by Keith Compton.
The Liberal Party also endorsed a candidate, making the seat a three-cornered contest.
Duncan established a strong electoral record, making Lismore a safe seat in 1968 and being elected unopposed in 1971.
The National Party, however, did not oppose his election in 1984 and he won his seat easily as an independent.