I've been going through the code and history...I am not really sure why this was never added to the keymap definition options. This menu option has been in the code forever, but no keybind mapping option was ever setup for it. My guess is that the duplicate function originally had some unusual implementation that made a keymapping problematic, but I do not see any reason we could not add this as a mapping option now. I will see about putting this in for a future update.