I have never understood nested duplicates. I agree with Anna that it sends a pretty bad picture of the site, and it is completely unnecessary. When voting to close a question A, it is perfectly clear that the proposed duplicate (call it B) is itself a duplicate (of, say, C).
There's a few situations that could cause this:
- C contains answers that answer the concerns in A. The correct course of action is to mark A as a duplicate of C, directly.
- B contains answers that address the concerns in A, and the answers to C don't, but those concerns are indeed in the question in C. B and C should probably be merged - this is done by moderator action through a flag.
- B contains answers that address the concerns in A, and those concerns are not relevant to the question in C. B and C are likely to not actually be duplicates, so B should be reopened.
There's no situation where it's advantageous to have a chain of duplicates. Moreover, doing that deprives C of having A on its
Linked sidebar, which makes it harder to gauge its importance as a frequently-duplicated question. If the intent is to make B known to the readers of A, it is still in the
Linked sidebar of C and one can still drop a 'Related: ...' comment on A if really necessary.
Seriously, I don't get nested duplicates.