I recently marked a question (subsequently closed as not constructive) as an exact duplicate of another, closed question.
Another user questioned this as being nonsensical. While the question wasn't an exact duplicate, the response to it would be.
Is there any reason why not to mark a question as duplicate of an already closed question?
Even if you choose "off topic" as the reason to close, I would still at least point out the duplicate if you know about it. There are two goals to closing a question: keeping the question from getting further attention, and educating the asker so they'll be a better contributor. Pointing out the duplicate is a nicer way of saying, "Next time, look to see if similar questions have been closed."
There are two ways I see it not being helpful:
It's not actually a duplicate. But I mean this in a pretty loose manner: if the original question covers the same ground as the new question, starting with the same set of assumptions, it's a duplicate. If the question is asking about a different problem but still has the same answer as the original, it's not a duplicate.
The original question is worse than the new question. Generally, this happens when the older question was terribly worded, contained no useful information, or was otherwise unsalvageable whereas the new question was borderline or had far more useful answers, and has a chance to be reopened.
The second one requires a little bit of a judgement call, but I think it comes down to effort expended. If you took a look at both questions side-by-side, which one has the least amount of work ahead of it to get it to meet our quality standards? If it's the older one, close the new question as a duplicate. If it's the new one, close it for one of the other reasons.
But in many cases, this is largely a pragmatic decision: in a perfect world, we wouldn't have unsalvageable questions lying around, and all questions that have a chance to be reopened with a heroic edit would, in fact, be open.
But due to questions slipping through the cracks or lack of community effort, we still do have a lot of the former and salvageable questions that simply aren't.
So it's okay, in some cases, to rely on a closed question as a resource particularly for closing as an exact duplicate. If the question is bad, but there happened to be an existing question that—while also bad—actually answers the new question, I say point it out.
I swear I just saw this come up somewhere else... but anyway, it doesn't terribly matter either way. If the duplicate was closed, that means it doesn't belong on the site so it's likely that the new question doesn't either.
There are some exceptions (like, the duplicate is closed but is on topic and might get reopened with edits), but in general you can really go either way and there's nothing inherently wrong with voting to close as a duplicate of a closed question.
Let's assume Q2 to be an exact duplicate of Q1, which is closed for any other reason. We close Q2 as a duplicate of Q1. And then Q1 is edited and reopened.
Assuming the system works, Q1 wasn't closed for a trivial reason, if there was a chance to salvage it, we would have instead of closing it in the first place. So any edits that led to reopening must have been substantial.
So at this point we have a substantially altered open Q1 that might or might not be an exact duplicate of Q2, which is still marked as duplicate. I think the "might not be" chance is enough to lead to messy situations. And that's what I think is inherently wrong with voting to close as a duplicate of a closed question.
In my mind "exact duplicate" is a weak close reason, and all the others are strong. If a question should be closed for two reasons, the stronger one should be chosen.
And since comments are ephemeral, I was the one questioning closing the question as a duplicate.
