He played college football for the San Francisco State Gators, earning All-American honors in Division II in 1978.
Born in San Francisco,[1] Duncan attended Jefferson High School in Daly City, California.
As a senior in 1978, Duncan was only a second-team all-conference selection in the Far West Conference, though he was named a Division II All-American.
Only the cornerbacks remained the same in the secondary, with Duncan and Bob Gregor replacing Pete Shaw and Mike Fuller as safeties.
[9] In week 2 against Detroit, Duncan stepped in front of the Lions' Freddie Scott and intercepted a pass from Gary Danielson at the Chargers' one-yard line with one second remaining in the game, preserving a 28–23 win for San Diego.