Originally Posted by cklammer
Calculate for each every epub file a CRC32 checksum and store it and the file size of the epub file directly after download. Now when you download a new epub you just need to compare tboth he CRC32 and filesize of the new epub with your database records. If you find a match than it is very, very, very likely that one or more of the epubfiles in the collection and the newly downloaded epubfile are identical then you just have to act accordingly (warn the user or whatever).
I think the best idea would be to use the identifier of the ePub. Idential identifier = identical epub.