As the title says.
We receive a lot of duplicate Ideas, so we do a lot of merging. When we merge, we also close the thread in order to funnel additional comments into the consolidated Idea. But that duplicate Idea can still receive upvotes.
Adding on that, the duplicate ideas still show up in the search, and that discoverability might lead to upvotes on those duplicate threads.
We use the upvote tracker to see who we need to follow up on for each feature, so having new upvotes on closed threads makes it difficult for us one reliable upvote counter/list for each Idea.