In 1924 Duncan made 4 appearances and scored 2 goals for the Irish Free State. The first two of these games were played at the 1924 Olympic Games. Other members of Olympic squad included Tommy Muldoon, Joe Kendrick, Jack McCarthy, Johnny Murray and Dinny Hannon. Duncan made his debut for the Free State in their first ever senior international on 28 May. He also scored the only goal in a 1–0 win at the Stade Olympique against Bulgaria. As a result of this win the Irish Free State qualified for the quarter-finals. On 2 June Duncan also played in this game against the Netherlands but the Irish Free State lost 2-1 after extra-time.