Nicknamed "Super Duper", he played 11 seasons for the Dolphins where his best years came while teamed with Pro Football Hall of FamequarterbackDan Marino and fellow wide receiver Mark Clayton, the other half of the "Marks Brothers" wide receiver tandem. Duper, who wore #85, was a 3-time Pro Bowl selection in 1983, 1984 and 1986. His best seasons were 1984, when he had 71 catches, 1306 yards and 8 TDs, and in 1986, when he tallied 67 catches, 1313 yards and 11 TDs. Duper had four 1,000-yard seasons, with the final one coming in 1991 at age 32, when he posted 1085 yards.
Duper was also a track star, he won in the finals of the 400-meter relay at the 1981 NCAA track and field championships at Northwestern State University, and from the 1980 Olympic trials finished seventh in the 200-meter dash and reached the semifinals of the 100. he competed in the 100 meters and 200 meters, posting personal bests of 10.21 seconds and 20.77 seconds, respectively.
Soon after retiring, Duper was charged with conspiracy and intent to distribute cocaine. He was acquitted one year later. Previously, Duper had been suspended in 1988 for 30 days by the NFL for violating its drug policy. In March 2013, Duper was charged for beating his 17-year old son in their Jacksonville home.