Open any image, then duplicate the background layer twice (Ctrl-J will work here)
Change the blend mode of the top layer (I don't think it matters which)
Merge down (Ctrl-E)
Ctrl-J now does not work.
You can right-click and select duplicate layer, or use the Layer menu, but the keyboard shortcut does not work.
This behavior was in v13 as well and I expected it to be fixed in v14, but no dice.