I agree that duplication is overkill. I only duplicated once, with a student interviewer, because I wasn't sure if her address listed on the school's directory was still current.
But I'm still really unsure about whether snail mail or email is better. I always felt more comfortable with handwritten snail mail letters--I agree with the PP that it seems more formal, and demonstrates more effort.
But I've also wondered if email might be better in cases where you know the ADCOM is going to meet shortly after your interview or the interviewer is going to write the evaluation ASAP. If that's the case, it makes sense to me to email, so that the interviewer has your enthusiasm for the school in mind as they write your evaluation.
Of course, when I was interviewing for law firm jobs, one interviewer told me he was really impressed when he saw the postmark on one interviewee's handwritten thank you letter--it was postmarked the same city as the firm was located in, even though the interviewee was OOS. That meant the interviewee wrote the thank you promptly and deposited it in a local mailbox before he took his return flight home. The interviewer said he was really impressed by that promptness.
Any ideas?