[1] Several of his classmates included men who would, like Duncan himself, eventually rise to general officer rank, such as John J. Pershing, Charles T. Menoher, Walter Henry Gordon, Edward Mann Lewis, Mason Patrick, Julius Penn, Avery D. Andrews, John E. McMahon, Ernest Hinds, William H. Hay, James McRae, Lucien Grant Berry and Jesse McI.

[2] He remained in this appointment until May 1918 when he was selected by General John J. Pershing, his classmate at West Point who was now in command of the AEF, to take over the 77th Division.

[3][4] As a result of his service in World War I, he received numerous decorations, including the Croix de Guerre with two palms and a star and status as a Commander in the Legion of Honor from France, status as a Companion of Order of the Bath from the United Kingdom, and the Army Distinguished Service Medal from the United States.

Arriving in France with the first contingent of American troops, General Duncan commanded in turn a regiment, brigade, and division with conspicuous success.

In the command of the 77th Division in the Baccarat sector his sound military judgment, energy, and resolution were important factors in the successes gained.