He was the first overall pick in the 1959 NFL draft by the Green Bay Packers, but played professionally for CFL's BC Lions and the AFL's Dallas Texans.
He moved with his family to Mason City before finally attending Roosevelt High School in Des Moines.
Duncan was a first team all-state quarterback for Roosevelt, leading the Roughriders to an undefeated season and a state title in 1954.
Duncan graduated from high school after the 1954 fall semester, and he was heavily recruited after choosing to play football.
As a mid-year graduate and due to freshman ineligibility, Duncan had to wait one and a half years to play, joining Iowa in the spring of 1955 but seeing his first action as a sophomore in the fall of 1956.
Duncan became very discouraged over being constantly berated by Iowa coach Forest Evashevski and being clobbered in practice by Cal Jones.
In a non-conference game against Oregon State, Duncan led Iowa to two fourth-quarter touchdowns and a 14–13 win after Ploen was injured.
[3] Duncan was terrific, helping Iowa lead the Big Ten in passing and the nation in total offense.
He later explained, "That was Green Bay before Vince Lombardi (hired two months after the draft), and Canada offered a lot more dough.
"[8] Duncan played two disappointing years in Canada before getting cut and then signing with the American Football League's Dallas Texans (now the Kansas City Chiefs).
He practiced with the Texans during the day and attended Southern Methodist University law school at night.