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It means that more than one book in the library had the same calibre database ID. This frequently happens when the calibre library is on a NAS/network drive, because many NAS implementations cannot handle renaming files/directories using hard links. One failure mode is to create the new directory but leave the old directory behind. Calibre restore sees these directory as books with the same ID. It can also happen when using cloud storage and running calibre on more than one computer at a time.
You should check that book, author Spufford_ Francis, title Red Plenty, to see if the cover and all the formats are there.
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