Duncan was born in
Washington County, Pennsylvania, on July 6, 1845. His parents were Jonathan F. Duncan and Agnes Harper Leeper. Duncan enlisted into the
Union Navy on June 26, 1863, in
Chicago, Illinois. He was assigned to the tinclad steamer
USS Fort Hindman at the time of his
Medal of Honor action in March 1864. He was mustered out of service on July 8, 1864, in
Red River, Louisiana. He reenlisted on March 4, 1865, and was discharged on July 13, 1866, in
Pensacola, Florida. Duncan married Lillian Jane Middlekauff (1860–1926) on November 4, 1879, in
Des Moines, Iowa; they had at least 6 children. In civilian life, he had a career as a physician. He was admitted to the
Northwestern Branch Soldiers Home on May 2, 1910. He died on March 27, 1913, and his remains are interred at the
Wood National Cemetery in
Milwaukee,
Wisconsin. ==Medal of Honor citation==