I ran into an issue where I was trying to bring in a file through Obok with a title that was already in my library. The book in my library with that title is a different book (different author). It seems like Obok only does a title match to determine if it's a duplicate and doesn't check the author/ISBN/etc.
Workaround:
Rename old book, import new book, and rename the old book back to the same title to keep both.
Calibre 8.10
DeDRM 10.0.9