(written from a Production point of view)
Duncan was the stunt double for Greg Morris (father of Phil Morris) on the television series Mission: Impossible, which was also produced by Desilu. He also made acting appearances on the show (including episodes with Leonard Nimoy), in addition to guest appearances on such shows as The Mod Squad with Clarence Williams III and Tige Andrews, Then Came Bronson with Robert Hooks, and The Odd Couple (with Bill Quinn, Garry Walberg, and Teri Garr).
Throughout the 1980s and 1990s, Duncan appeared on such television series as Knight Rider (with Lawrence Dobkin), 7th Heaven (starring Stephen Collins and Catherine Hicks, in an episode with Graham Jarvis and Raphael Sbarge), and The Pretender (with Paul Eiding, Gregg Henry, and Christopher Michael). In 2000, he was seen in the episode of The X-Files entitled "Redrum." He most recently appeared as a Supreme Court justice on Boston Legal along with regulars John Larroquette and William Shatner and fellow guest stars Bob Gunton and Jack Shearer.
He also appeared in a few films, the earliest being Zabriskie Point (1970) with Paul Fix, The Mack (1973) with George Murdock, and Hit! (1973) with Noble Willingham. He returned to films in 1995 a small role as a police officer in the drama White Man's Burden, which co-starred Brian Brophy, Bert Remsen, Bumper Robinson and Tom Wright. More recent film credits include Essence of Echoes (2002) and Taphephobia (2006).