How about a feature like in Safari where users can use a hotkey to duplicate tabs. I suggest Alt + Shift + D on Windows and Option + Shift + D on Mac. I would be cool if this worked for whole windows.
@knowbane3760 thanks for the request! You should already be able to assign a keyboard shortcut for Duplicate Tab via about:keyboard — let us know if it works on your end.
And to be clear, you are also looking for a hotkey to duplicate an entire window?