5.0 is a bit old, 5.1 is the latest. I don’t remember what hotkeys have changed and when, but IIRC 5.1 already has a bit different UI for key assignment. In any case the nightly builds have “Duplicate” action in “Common” section and you can assignit to e.g. hotkey “D”. It works in eeschema, too. If it’s not in 5.1 it will be later in 6.0.