Well emphasis on might but Clemente had a huge impact on the sport of baseball.
Like Gehrig, much of Clemente's impact came because he died young.
He was a great player before that, but he did not become a real bonafide legend until the 1971 World Series and then his death just over a year after that.
I mean, he was not even the best player in his league at his position, and was not even the second best until Frank Robinson went to the AL after 1965.
Conversely Duncan is the least dynamic all time great player in NBA history.
I'd go with Clemente only because baseball is much bigger than the NBA, especially during Duncan's career.