The thing is, for an ISBN, that is one edition, by one publisher and one can easily have the same text under 3 different ISBNs from one publisher (hardcover, trade paperback, mass-market paperback).
I count 80+ editions of J.R.R. Tolkien's _The Hobbit_ at:
https://tolkienlibrary.com/booksbytolkien/hobbit/editions.ph...
granted some predate ISBNs, one is the 3D pop-up version, so not a traditional text, and so forth, but filtering by ISBN will _not_ filter out duplicates.
There is also the problem of the same work being published under multiple titles (and also ISBNs) --- Hal Clement's _Small Changes_ was re-published as _Space Lash_ and that short story collection is now collected in:
https://www.goodreads.com/book/show/939760.Music_of_Many_Sph...
along with others.
Hmmm, yeah, ISBN isn’t great for this. Is there a good way to deduplicate the books by their contents?
LoC or Dewey Decimal with author and title (and edition?) should work.
I wish there was some better book cataloging/organizing scheme --- the Online Books Page uses LoC:
https://onlinebooks.library.upenn.edu/subjects.html
and is the most workable of the indices I've used.