CALM is a plug-in,
https://www.mobileread.com/forums/sho...d.php?t=265947
CALM consolidates all metadata, including custom columns.
The metadata includes page count, language, file size, any custom columns you have created, plus all of the other standard columns.
I asssume you have run all of your EPUBs through the 'Modify EPUB' plug-in prior to running the 'Count Pages' plug-in with almost all (but not all) of the checkboxes 'checked'.
When visually comparing them in CALM, if the file sizes or page counts are vastly different, or the languages are different, then you know they likely are not true duplicates.
Run the 'Extract ISBN' plug-in for all of them. If their ISBNs are identical, that is a clue that they are identical.
If their ISBNs are different, then run the 'Library Codes' plug-in to determine their Dewey Decimal Codes. If their DDCs are significantly different, then that is a clue that they are not true duplicates.
If the above does not work for you, then comparing their texts side-by-side is warranted. Since they are in different physical Calibre libraries, that is easier said than done. With some creativity, you can figure out how to ergonomically do so.
CALM has a 'Library Tools' Tab that can be used to delete books from their Source Library, or to copy from one Source Library to another Source Library. You can also standardize their metadata via the 'Metadata Tools' Tab.
Good luck.
DaltonST