Officer Duncan began his career in law enforcement in 1990 with the Gregg County Sheriff's Office. Officer Duncan transferred to Rusk County Sheriff's Office in 1993, and worked for 13 years in patrol, Mental Health Peace Officer, and Criminal Investigations Division. In 2002, he joined the Kilgore Police Department, serving in patrol and CID. In 2009, he returned to Rusk County Sheriff, where he served as Chief Deputy. He joined the Longview Police Department in 2012. Prior to being assigned to the POST unit, Officer Duncan served in patrol, working south and central Longview. He is also a Field Training Officer with the department. He is a Texas Master Peace Officer.