Less than two weeks after he finished 6th at the 1968 Indianapolis 500, Duman was at the next race of the USAC Championship season, the Rex Mays Classic at Milwaukee. Duman started 14th in the 24-car field, driving the Central Excavating Gerhardt Turbocharged Offy. On the third lap, the cars of Bay Darnell and Norm Brown reportedly got tangled in turn one. Both cars slid towards the outside wall, collecting Duman in the process. Duman's car tipped over and began barrel-rolling before it flew into the catchfence cockpit first. After Duman's car bounced back down the track, both Darnell and Brown's car slammed the outside wall and the three cars came to rest on fire. Darnell and Brown suffered minor burn injuries, but Duman was killed instantly from massive head injuries when the cockpit of his car struck the catchfence. His helmet had been shattered and pulled from his head. Eight spectators were also injured by the flying debris which cleared the catchfence.