Recently, Plex keeps importing old movies in my library as new, and I've been trying to figure out why that's happening. It's been especially annoying, as it is discarding the custom posters and other metadata that I've entered for the movie, and wiping out a lot of work I did to make the metada nice and clean. However, I think I may have figured it out.
The network mounted drives on the home server will occasionally go offline, and I have a script that detects that, and shuts down a few apps that need the drives, re-mounts the drives, and then re-starts the apps. Plex is one of those apps. Plex also has this setting where it will detect missing media, and "clean up" the library, deleting those media entries, if you have "Automatically Empty Trash" checked. I didn't think I had that checked, but it turns out I did.
So, I suspect somewhere in the few seconds between the drive going offline, and the script detecting it and shutting Plex down, it was automatically deleting a bunch of media, and then when Plex re-launched a minute later, it would then see the media on the drive and re-import it as a new movie or whatever kind of media.
This is my current theory, anyway. I have not matched back all of the cryptic DELETE IDs in the logs to human readable media names as yet, but since I turned off that Empty Trash setting, things seem to be more stable.
As to why the network drives keep unmounting themselves, I haven't quite figured that out yet, but seems to be happening more often recently. Might be the Synology RAID warning that it's almost out of space, and for whatever reason (which I've seen before) it seems to get unstable in that state. More investigation needed, but hopefully this helps.