Duignan was still eligible for under-21 grade when he was added to the Offaly senior team in advance of the 1987-88 National League. He made his debut for the team on 11 October 1987 when he lined out at left corner-forward in a 3-11 to 0-11 defeat of Laois. On 6 March 1988, Duignan was at midfield when Offaly secured the Division 2 title after a 1-13 to 3-04 defeat of Down. He was later included in the Offaly panel for the Leinster Championship and made his first appearance on 19 June 1988 when he scored two points from midfield in a 2-13 to 2-10 defeat of Dublin. On 10 July 1988, Duignan won his first Leinster Championship medal after again lining out at midfield in a 3-12 to 1-14 defeat of Wexford in the final.