Command-J duplicates the entire current layer, or a portion of the current layer, if there’s a selection. It’s a great feature, and one I use daily.
For some strange reason, Command-J doesn’t do anything when there’s two layers or a layer group selected. Why? I realise there’s probably a decent technical reason for this, but a context sensitive “just dupe it” command would be really handy, even if it’s actually several different methods behind the scenes.
Is now a good time to mention that the Layer menu already contains an item that does most of these things, but it doesn’t have a keyboard shortcut by default? Pre-emptive strike: Assigning a keyboard shortcut wouldn’t help, as the menu item doesn’t duplicate selections within a layer (Command-J does).