Duncan received the country's highest award for bravery during combat, the Medal of Honor, for his action aboard the USS Fort Hindman on March 2, 1864.

He was assigned to the tinclad steamer USS Fort Hindman at the time of his Medal of Honor action in March 1864.

Duncan married Lillian Jane Middlekauff (1860–1926) on November 4, 1879 in Des Moines, Iowa;[8][b] they had at least 6 children.

[10] He died on March 27, 1913, and his remains are interred at the Wood National Cemetery in Milwaukee, Wisconsin.

Carrying out his duties through the entire engagement, Duncan served courageously during this action in which the Fort Hindman was raked severely with shot and shell from the enemy guns.