Corporal Duncan was a member of Company L, 3rd Battalion, 34th Infantry Regiment, 24th Infantry Division. He was taken Prisoner of War while fighting the enemy near Taejon, South Korea on 20 July 1950, forced to march to North Korea on the "Tiger Death March", and died while a prisoner at An-Dong, North Korea on 8 April 1951. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Duncan was awarded the Combat Infantryman Badge, the Prisoner of War Medal, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.