Duncan enrolled in Ventura Community College in 1964, playing both football and basketball. He was named All-Western States Conference (WSC) in football and captain of the basketball team in 1964. He played on the 1965 WSC Championship football team and was named an All-WSC first-team receiver, catching a school record 56 passes.
Drafted into the U.S. Army in 1967, Duncan served a year-long tour in Vietnam. He returned to the University of Tulsa in Oklahoma in the spring of 1969 where he played two seasons of football. Duncan was selected a Sporting News All-American as a punter in 1970 and was chosen as team co-captain and tied the school record punt of 87 yards that year.
Duncan was drafted by the Minnesota Vikings in 1971. He played for the Green Bay Packers from 1971-72, until a knee injury sidelined his career. Duncan is now a professional personal trainer and can occasionally be seen on television, commercials and film.