After completing a public school education, including graduation from McMinnville High School, he attended Linfield College in McMinnville, Oregon. Duncan served in the United States Army, remaining active in the reserves, ultimately attaining the rank of Colonel. Duncan graduated from Idaho State College, earning a B.A. in 1960, and began a long career in education. He would ultimately earn several graduate degrees, including an M.Ed. in Education Administration from the University of Idaho, a PhD in Public School Administration from the University of Oregon and an M.B.A. in Labor Management from the University of Portland.