When reviewing close votes in the new /review, the UI has a bit of an ambiguity regarding duplicates. More specifically:
I assume "Original" refers to the question being voted on for closure, as it is the default view when reviewing this situation. However, closing it as a duplicate of another question implies that the other question is "original", and the question in question is a duplicate.
Perhaps the tabs could be relabeled as "Possible duplicate" and "Duplicate of..."? (or perhaps something better, but I can't really think of anything off the top of my head just now)