Quote:
Originally Posted by kovidgoyal
1. Why is it catastrophic to distribute EPUB books with the same id?
2. If you specify an ISBN in the metadata, calibre will put that in the EPUB file. Having and additional calibre specific identifier is no problem at all.
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The main identifier should be unique since it can be used internally by a reading system to identify books, or for things such as annotations/bookmarks to globally identify books.
Of course multiple identifiers are fine, but:
1. You don't always have an ISBN
2. You're using the calibre id as the package identifier