Just an avid reader. Mostly SF/Fantasy, some hobbies, paranormal, urban fantasy and lighter, fluffier things.
Ways I tried that appear to work but duplicates show back up when I look later, sometimes with even more of them (no, not importing more data files and haven't since 2015):
1. Go under "your books," click power edit icon (blue icon with yellow lightning bolt, aka batch edit), click Miscellaneous tab, click Find ISBN Duplicates, select books to delete, click Delete Selected Books.
2. "Your books" normal (not power/batch edit) and eyeball for duplicates to delete by clicking the red "X" (honestly, if this worked not sure I'd put in the effort to get rid of the thousands of duplicates I have one by one but it's bloody weird it won't work):
3. Go under "Stats/Memes" and click Work Multiples then right click open each book edition I want to delete and click the "X" (sometimes this method works, sometimes the duplicate comes back; the first in screenshot isn't a duplicate because I do collect editions of it):