in 1897, duncan became a worker for the state of illinois, serving in the office of the superintendent of public instruction (the predecessor of the current illinois state board of education). in addition, duncan entered the illinois national guard in 1902; assigned to the 8th illinois, he was commissioned as an officer. 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.