I've got 1050 books all catalogued in LibraryThing. I find it quite worthwhile – I can search my library, useful if you have an extensive reference collection on a particular topic. And if I'm out at a bookshop wondering if I have a book already, I can check! :)
A true collector would buy the book regardless. The duplicates are part of the character of the collection. If you like a book that much, gift the extra.
