Why does this happen?
Lower reputation users (< 3k) cannot vote to close, but they can flag posts for moderator attention, which includes a 'Duplicate' option.
I bet you a thousand reputation points that the post in question was flagged in that manner.
This is what it looks like for a lower rep user (like me, here on SO):
Note that it looks identical to the 'vote to close' dialog, but instead I'm informing the moderators here. It makes perfect sense to have the wider community review these to help out.