Duncan has been a member of the Nashville Bluegrass Band since 1985. He also works as a session musician and has played with numerous well known performers including George Strait, Dolly Parton, Guy Clark, Reba McEntire and Barbra Streisand. In 2006, he toured with the Mark Knopfler–Emmylou Harris Roadrunning tour, and he appears on their All the Roadrunning and Real Live Roadrunning albums. In 2008, he joined Robert Plant and Alison Krauss on the tour for their critically acclaimed album Raising Sand. He appeared on Transatlantic Sessions Series 4 broadcast by the BBC in September/October 2009. In 2011, Duncan collaborated with cellist Yo-Yo Ma, bassist Edgar Meyer, and mandolinist Chris Thile on the album The Goat Rodeo Sessions. In 2013, Duncan accompanied Diana Krall as a sideman on her Glad Rag Doll tour.
Duncan has received numerous awards. As a member of the Nashville Bluegrass Band, he won the Grammy Award for Best Bluegrass Album in 1994 and 1996. Duncan was named the Academy of Country Music Fiddle Player of the Year for 1996, 1998, 1999, 2001, and 2004, and Specialty Instrument Player of the Year for 2006.
In 2015 Duncan appeared on one of the tracks from the Jericho Summer album, Night Train which had been recorded in Berry Hill Studios, Nashville.