The only logical scenario that would not involve direct user input would be if you have a watched folder. You could have a situation where you catalog a file, tag it with data using the desktop client and don’t embed that data, so it’s only in the Portfolio catalog. Then somebody removes the file out of the watched folder and Autosync deletes the record from Portfolio, and then the file gets put back on the server, so recatalogued but would now be missing the custom tags you previously added.
It is possible to catalog the same file twice if the ‘path’ to it is different, even for exactly the same file. Had a customer just this week with paths using \servername\f$\foldername\ , that includes therefore the ‘f$’ hidden share for assets. You can also catalog directly from \servername\foldername\ and thus end up with two records pointing to exactly the same file in the same location on your server! So I helped them with cleanup so that they only have one record now pointing to the standard path without the hidden share element. The Windows Desktop Client can search for ‘Duplicate Items’ though if you want to see if you have more than one record where they share the same filename.