I summon thee back from the dead!
Actually, this popped up while searching for something else, and in case it pops up for people looking for this subject:
The the “Duplicate” and “Copy/Paste” options might be unavailable in the Content Browser’s context menu, especially when dealing with certain asset types or certain situations. This is because Unreal tracks assets and their references behind the scenes, and copy/paste may break the asset management systems. The same issues could come from copy/pasting on the File Explorer level.
UE is large and complex and you should let the system manage itself as it’s been designed.
It’s also worth researching “redirectors” and having a better idea of what happens when you moved something within your project.