ChaoticAdventure,
I suspect you've left out part of your config.
As shown, you should end up with "Harry Potter, Harry Potter/Hermione Granger", not "Harry Potter, Harry Potter, Hermione Granger". And what's really there is two strings: "Harry Potter" and "Harry Potter, Hermione Granger" -- that's why FFF isn't de-duping.
So I suspect you have a replace_metadata line that's changing the "/" to "," in
characters. (Or both / and &.)
If you change the "," to "/," in that line, it will split into two entries and be de-duped in FFF. (
Wiki page.)
That should work for both regular and anthology books.