For example for the following book uploaded by mtravellerh:
https://www.mobileread.com/forums/showthread.php?t=43041
Quote:
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<dc:identifier opf:scheme="calibre" id="calibre_id">736</dc:identifier>
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This is NOT a unique identifier and while it maybe be ok for files that you convert for yourself, it is pretty catastrophic to distribute files with such an identifier. You might have a lot of books uploaded here on Mobileread all sharing the same identifier because of this problem.
My advice would be to assign a UUID or another unique identifier to every book available in the ePub section to fix this problem.