For questions that were closed due to an issue in the question (like being too broad), people can take the closure reason into account when voting. I think many such questions don't gain new upvotes after being closed, though we do need to consider the case where people disagree with the closure and think the question is fine.
Duplicates, however, are special. There can be several ways to ask a question, and we should not penalize someone for not guessing the way it was originally asked and therefore not finding it on a search. We have differences in vocabulary, level of jargon, assumptions about background, even specificity and framing of the question... it's quite possible for somebody to ask a good question, one worthy of upvotes, that is also a duplicate. This is why the algorithms that delete (some) closed questions don't delete duplicates -- the duplicates serve as additional way-finding, so now there are two ways for somebody searching the site to get to the answers.