11:29 AM
There is a query for duplicates of deleted questions.
Perhaps it would be nice to have a query for questions which are closed as a duplicate and the duplicate target is itself closed: Should a closed question be reopened if it gets a duplicate?.
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This question How do you prove that \$n^2-1\$ is composite when \$n>2\$? was closed as a duplicate. That was the right outcome, as opposed to, you know, "off-topic." But the question it's a duplicate of is closed! Prove that there doesn't exist any integer \$x \ge 3\$ such that \$x^2-1\$ is prime. At le...

I have tried this simple query: Duplicates of closed questions.
Maybe it could be useful to have also a query which omits those ones where the duplicate target is closed as a duplicate. (And only leave other close reasons.)
I am not really sure how to do that. Should I use PostLinks JOIN PostHistory and check for CloseReasonId? I'm guessing that this would be rather slow, since PostHistory table is probably bigger than Posts.

4 hours later…
3:37 PM
Since in most cases found by the above query, the target is closed as duplicate, I'll add a link to this post on meta (which is - to some extent - related): Should questions ever be marked a duplicate of another duplicate?
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Should questions ever be marked a duplicate of another duplicate question? Specifically, I'm looking at these three questions, each one marked as a duplicate of the next: ?+?+?=30 (1,3,5,7,9,11,13,15) What three odd integers have a sum of 30? What five odd integers have a sum of 30? Shou...