Tried it. Good stuff!
I have several intentional duplicates -- books in French and English where the title is not translated. I await the non-dup marking with bated breath.
A suggestion: an option to see the union of all groups, sorted by title. Use case: when I run similar title/ignore author, I get 6 groups where each group contains two books. I could tell at a glance if these are correct by seeing them all, sorted by title. A variant of this would be to show me all the books and have 'change group' use highlighting the books in a particular group.
Behavior oddity: The first time I press next-group after having run the finder, nothing happens. I need to press it again. I suspect that this happens because the first group is already displayed, and the first press displays it again.
Minor suggestion: I would prefer to see the tests listed with title first. Rationale: all of the tests do something with the title, but some ignore the author. In my mind, that makes title 'superior' to author, so I want to see it first.
Minor suggestion: add a 'next group' action so I can add it to my toolbar/menu bar.