[ownedbycats]

The number is the Calibre ID - it's how Calibre's database identifies a specific book, since title and author can change (or be the same as another book).

It sounds halfway like your library folder got re-added to itself but I can't think of any way to invoke that without noticing.

Is this Calibre library on any sort of NAS or cloud storage such as Dropbox or OneDrive? You're not supposed to do that, and we get at least a post a week from people who've messed up their libraries by doing so.