Duncan attended Potomac High School, in
Oxon Hill, Maryland. Playing at both receiver and tailback for Potomac's football team, he accumulated 2,209 yards his senior year, including 958 yards receiving and 808 yards rushing, and was named a high school All-American by
Football News and the Maryland Player of the Year by the Washington Pigskin Club. Along with teammates
Willie Gault,
Anthony Hancock,
Lenny Taylor and
Tim McGee, he helped create the school's reputation as "Wide Receiver U." He played sparingly in 1979,
redshirted in 1980, played as a defensive back in 1981, and played primarily as a reserve receiver in 1982. In 1983, however, he led the team in receiving with 33 catches for 640 yards and six touchdowns. He caught touchdown passes of 80 yards and 57 yards in Tennessee's 41–34 win over Alabama, and his 85-yard touchdown catch against Vanderbilt remains the third-longest in school history. He attracted close attention from scouts for his workout performance at the 1983
Blue–Gray Football Classic. == Professional football career ==