Though it may not be the most diverse campus in the world, Duke does have a large and thriving community of students from almost every ethnic background. Sometimes these "diverse" groups self-segregate (particularly through race-specific clubs or Greek organizations), but, on the whole, the ethnically and geographically diverse student body meshes well.
Economic diversity at Duke is a bit of a different story. In this regard, the Duke population is noticeably homogenous. The majority of students hails from the upper-middle class, as evidenced by the rich, preppy style sported by many on campus. This is not to say that everyone owns a yacht, plays golf and squash at a country club, and will come into a trust fund at graduation, nor that students are obnoxious and eager to show off their wealth—but you will want to be aware of this social makeup before you come to Duke.