He was a member of a long-established African-American family of Springfield, Illinois; his father was a grocer and his maternal great-grandfather, barber William Florville, had been a friend of Abraham Lincoln.

When the 8th Illinois was called into national service during the Pancho Villa Expedition into Mexico in 1916, Duncan served as a major on the regimental staff.

[1][4] Despite (or perhaps because of) his service as an officer in the Illinois National Guard, Duncan was a prominent victim of the Springfield Race Riot of 1908.

[4] After the American entry into World War I, in April 1917, the 8th Illinois, still in national service, was renamed the 370th Infantry Regiment.

This was a significant achievement due to the segregationist attitudes of President Woodrow Wilson and his secretary of war, Newton D. Baker.

While serving on the Western Front against the German Army, Duncan was awarded the Purple Heart and the French Croix de Guerre for gallantry in action.

In February 2017, authorities announced the discovery of a post-World War I 8th Illinois collar disc, a service unit insignia artifact connected to the date of Col. Duncan's command.