Digging in the SEDE, I've found 3 question pairs being each other's dupe. Namely:
Today we can't close a question as a dupe, if it would create such a dupe circle. But it is a new development in the software, and it did not affect the already existing circles.
In order to make the question catalog more consistent, I would suggest to do something with them:
- Vote the better to reopen (note: chronological order doesn't matter, nothing forbids a question to be a dupe of a later posted one).
- Vote to delete one of them (note: keep the relevant rules, particularly avoiding the deletion of good answers).
- If some exceptional circumstance would require it, then flag for mod intervention (merge suggestions, or dealing with multiple originals).