so there is no way around it... one has to have a main library, and a clone library (Adult Library)?
I'm confused.
If you have two libraries, they can't have the same unique identifier. Ever. Even if you move them to other machines, it is a bad idea. If they ever "meet" on the same machine (via sharing or whatever) then bad things happen.
The way you create a new copy of an existing library, and give it a new GUID, is to use MC's built-in clone tool.
So, if you screwed this up at some point in the past, and you NOW have one machine that has two libraries with matching IDs, you need to fix it. Say you have Library A (the original) and Library B (the dirty stinking furry porn), and the thumbs/settings are getting all screwed up, do this:
1. Clone Library B to a new Library and call it B-Temp. Put it in a
new location somewhere.
2. Switch to it and make it the active library (if it isn't already).
3. Delete the original Library B. You can also delete the original library files off of disk if you want (MC doesn't do this for you, so you have to track them down in Windows Explorer and trash them).
4. Rename Library B-Temp to Library B. If you really, really want it back where it was originally, you can instead clone it back from Library B-Temp back to Library B (and put it back in its original location, but make sure to delete the stuff that's there first). Because I'm paranoid, if I was doing this, I'd make sure to close out of MC completely and re-open before starting. This is probably unnecessary.
5. Enjoy, now you have two libraries with separate IDs. Don't mess it up again.
PS. If you directly access Library B on other copies of MC (not via the library server, but by loading the library directly), you'll also need to delete the Library out of those copies of MC and re-add it once you're done with the above steps.