Duncan owned the Double D Ranch, a 5,000-acre (20 km) hunting property in Texas. Duncan was a Safari Club International member who has been given numerous awards for his hunting trophies. In 2006, he received the SCI World Conservation Hunting Award, which requires a hunter to have hunted on six continents, and have received SCI's 13 "Grand Slam" awards, 22 "Inner Circle" awards, the Fourth Pinnacle of Achievement Award and the Crowning Achievement Award. Duncan has 407 entries in SCI's trophy record book, including records for killing lions, elephants, rhinoceroses, a polar bear, and other animals.