Duncan was a Lieutenant in the U.S. Army, Expeditionary Forces during World War I. He was attached to the 11th Company of Engineers. He served in five offensive and one defensive sectors. After his discharge from the U.S. Army, he stayed in France, married a French woman and opened a gymnasium in Paris. He was also the caretaker and manager of the American Military Cemetery at Suresnes, near Paris and corresponded with many American mothers, informing them of his visits to their sons graves.