In 2000, Duncan participated in the American men's basketball championship as a member of the American basketball team and helped the team qualify for the 2000 Sydney Olympic Games. Later, due to a knee injury, he could not participate in the 2000 Sydney Olympic Games.
In 2004, Duncan entered the "Dream Team 4" and represented the United States in the men's basketball competition of the 2004 Athens Olympic Games, winning the bronze medal.