What should I do when I find circularity among duplicates?

Occasionally, when trying to vote to close some question, you get the error message: This closure would result in the 'duplicate of' navigation only leading in a circle.

AFAIK the system should prevent from occurrences of loops between duplicates, but they still happen for various reasons.

What should I do, when I find something like this? Should I flag some of the questions in the cycle when I see this? Should I post on meta that there is a problem like this?