I went suit without tie. Some others that interviewed the same day as me wore business formal. Wear what makes you feel comfortable because I don't think it makes a difference, I know at least one of the other guys admitted with me went more formal. The interview is low stress but from what I've been told very important. The second-year student is really looking to see how you'll fit in to the program and contribute to Team Fuqua. If you've put a lot of thought into your essays (Why Duke?) and understand the school's culture and why you want to be a member of Team Fuqua it will come out naturally and you'll do fine. The Team Fuqua concept isn't a marketing slogan. The entire school lives and breathes it and you'll see it when you visit. People are genuinelly interested in others (and expect others to act the same way), so if you come across as arrogant or selfish I imagine that will hurt you in the interview.
Fuqua ended up being my only completed and submitted application. If you reach out to current students many will go above and beyond what you would expect in terms of help, I briefely considered hiring a consultant but after talking to some Fuquans I realized it wasn't necessary. For those looking to apply next year, if Fuqua feels right and is your number one choice definitely do the Early Action round. It's awesome being able to move on to the transition phase before Thanksgiving.