This may be too late for the OP, but here are my thoughts:
I’m a Harvard alum and have been interviewing for about 17-18 years now. I’ve never been asked to re-interview a student. However, I have been asked by the admissions committee to “fast track” a couple of interviews— meaning, “please interview this person as soon as possible.” My interpretation of this is that they only ask this type of thing when the student is being strongly considered and they need that information to make a decision.
That being said, I would suspect that Harvard asking for a second interview is a distinctly GOOD thing. All of us interviewers are VOLUNTEERS who have real lives and real jobs. Interviewing— and especially spending several hours writing up each interview report— is very time consuming. In fact, in our area, we don’t have enough volunteers to interview every single applicant, though we all work very hard to try to do that. Given all of that, the fact that they are asking for a SECOND interview for you should be taken as a positive— they think highly enough of you to devote time and resources to interviewing you AGAIN.
Put your best foot forward! It’s a good thing!