So low-rep users could help closing duplicates without flagging. No other close action of course, that would render the rep-requirement useless.
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I like David's answer, but let's add some mechanical concerns.
There's one fatal flaw in that auto-comments are neither guaranteed nor consistent. If the post already has a comment that links the target question, then no comment will be generated. Furthermore, the auto-comment can be modified by the closer, sometimes to specify additional information. This gets pretty problematic as sometimes we'll link more than one question when the duplicate has been particularly common.
Which means that this is subject to a lot of error. You'll have to basically parse out links in comments that correspond to a duplicate vote, regardless of who wrote them. Consider...
People find that related question useful, they upvote it because it is a useful tangent. One person thought that it was actually a duplicate, though, and casts one vote. Suddenly those upvotes turn from noting a good link to closing the question. People are already furious about incorrect or "spurious" closings from 3k users, let's not amplify this issue.
Another issue is that 3k close voters, if their judgment occurs but they realize they weren't correct, then they can vote to reopen the question. However, comment upvoters lack any capacity to reverse their decision.
That would assume that apparent duplicates are in fact duplicates, which isn't always the case.
Whether something is a duplicate is not always obvious. Something that looks like a duplicate may not be (for example, a question about the meaning of
Further, if I vote to close as a duplicate nowadays, I have to specify the question it duplicates, and the close box shows me that question. I believe this is a good thing, and it won't come up as part of a comment upvote.
Nor do I see an actual problem here. At least on SO, there's plenty of 3K+ users to close duplicates. I don't see duplicate questions staying open all that long. There may be a SF or SU problem, but I'd have to hear from those groups to find one (and this group is MSO, after all).