Sometimes I discover that "old" MO questions are duplicates of even older MO questions.
- Embarrassingly, I found out that my own question: Formula for number of permutations less than a given permutation in weak order is the same as a question from 5 years earlier: Principal Order Ideals in the Weak Bruhat Order.
- This question: How do I determine the number of "second degree" derangements? (which is actually relatively recent) is a duplicate of this question from 7 years ago: Number of Permutations?.
Now, I can vote to close the newer questions as duplicates (which I've tried to do)- but because these questions are not active, they are very unlikely to attract enough votes for this to work.
I wouldn't want these questions outright removed, of course, since often the duplicate questions contain non-duplicative answers.
In fact, what I have done so far is put a comment (in both of the questions) linking the two questions which are duplicates. This should help somebody who discovers the question later and might want to see all the various perspectives on it.
So: what should be done in cases like these?
(A related question would be: are there ways to improve detection of duplicate questions, especially as the database of questions answered on MO continues to grow larger and larger. As you can see in these examples, the "original" questions are from quite a while ago.)