He is the writer and producer of Lewis & Clark: The Journey of the Corps of Discovery, a four-hour documentary broadcast in November 1997. The film attained the second-highest ratings (following The Civil War) in the history of PBS and won a Western Heritage award from the National Cowboy Hall of Fame, a Spur Award from the Western Writers of America, and a CINE Golden Eagle, as well as many other honors. He is the co-writer and producer of Mark Twain, a four-hour film biography of the great American humorist which was broadcast on PBS in 2002. His next film with Burns was Horatio's Drive: America's First Road Trip, about the first transcontinental automobile trip, which he wrote and produced. It won the prestigious Christopher Award and a Telly Award.
In politics, Duncan served as chief of staff to New Hampshire governor Hugh Gallen; deputy national press secretary for Walter Mondale's presidential campaign in 1984; and national press secretary for Michael Dukakis's 1988 presidential campaign. President Clinton appointed him chair of the American Heritage Rivers Advisory Committee and Secretary of Interior Bruce Babbitt appointed him as a director of the National Park Foundation. Duncan now serves on the board of the Student Conservation Association, the National Conservation System Foundation and the New Hampshire Humanities Council.