Actually quite an old problem

When I started in SL I inadvertenly got two Notecard folders. I put up with it, and thought it would go away. With the new inventory stuff, I thought "Ahah! It will just go away!" but NO.

I cannot delete a Notecard folder. I have two. One is permanently empty and it must be taking up HUGE numbers of virtual electrons to keep it going. It could be responsible for every single bug SL has had for years. Any advice on how to remove this annoying wart?

Just a thought.
Test and see if you can drag one notecard into the other empty folder. And make sure you can open, edit, and delete that notecard. If works fine try dragging all your notecards into the empty one. Then delete the old one you had all your note cards in.

Just a though don't know if this will work. I'm just thinking maybe that one was some how a user created one where a person dragged a few notecards onto you. I don;t know, that is a strange one.

done everything. Notecard appears to be a protected folder. It was really early on - the noob island in fact - when the duplication occurred. Neither folder is deletable.

Had the same problem. File a bug report, LL will fix your inventory. Could take a lot of weeks though.

Mine was "Object" folder. Spontaneous duplication. Note that this is an evidence of some data corruption on your inventory image. Not too serious, but ask them to solve it.