When you use the Copy World button in the game settings, the game changes the world name of the copy to "Copy of <worldname>", but it doesn't update the levelname.txt file within the minecraftWorlds subfolder for the world. Levelname.txt does get updated if you change the world name in the UI, so I presume this is not intended.

I use a script to help me manage my world backups. The script can't access the world name shown in the UI or the generated folder name it's stored in, so it depends on levelname.txt to associate a world folder with a backup. In my case the script just gets confused, but the bug could cause someone with a similar script to overwrite one of their backups.

Steps to reproduce:
1. On Windows 10, use the Copy World button to make a copy of a world.
2. In File Explorer, browse to C:%UserProfile%\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\minecraftWorlds.
3. Look at the contents of levelname.txt in each subfolder. Observe that two of the subfolders have the same value.

Workaround
Since renaming a world in the Settings UI updates levelname.txt, simply rename the world after using Copy World.