Duncan has worked as a voice actor in a wide variety of games. He is perhaps best known as the voice of Atlas in God of War II. He has also voiced as Benjamin King, the leader of a criminal gang called Vice King from Saints Row. Duncan voiced another criminal in The Suffering: Ties That Bind. In SOCOM II: U.S. Navy Seals he plays SEAL Operative codenamed: WARDOG.
For 2013's Saints Row IV, Volition had originally planned to bring Duncan back to reprise his role of Benjamin King. However because of his death in 2012, the role was passed down to Terry Crews instead. Duncan is still credited all the same, with a respectful R.I.P. next to his name in the list of voice performers. The credits also play a small excerpt of him singing the song Just a Friend by Biz Markie.