You don’t need to wear a suit and tie. Just wear something modest and respectful. I’ve had an interviewee showing up in torn shirt and in shorts and flip flops. He apparently thought that he was a highly prized URM that no Ivy League would reject. No need to mention the result.
The most important advice I can give you is to be able to articulate your response to the question, “why Duke?” Even if the interviewer doesn’t ask this question, you should be able to articulate why you think Duke is the right university for you and no other. After all, Duke is Duke and nothing like any other schools. So what’s about Duke for you and no other colleges? If you can articulate that (likewise with any other colleges you’re interviewing for), then your interview would be a success.
The next most important advice I can give you is to be sure to have questions for the interviewer. When the interviewer asks the last question, “do you have any questions for me or about Duke?” there’s nothing worse than saying “no.” Use that question as an opportunity to convey why Duke is so important as the place to shape the next chapter in your life.