It looks like this issue has come up before, and been resolved, but it just happened again on Travel.SE. Perhaps this is another corner case? These two questions are closed as duplicates of each other.
I had the last vote on each, although in both cases, my vote was for "Unclear what you're asking", but the weight of votes in both cases were for the duplicate status. So if the existing checks are only in place at the time someone casts a close vote, that would explain how this one got in under the radar. Perhaps another check needs to be done when the question is actually closed, not just when close votes are cast?
And as @gnat points out, low-rep/logged-out users get a Redirect loop when trying to load either question.
NOTE: A moderator has already fixed this particular circular duplicate situation by closing one of the questions with a different close reason.