Probably restart your editor when that happens. If it still doesn't work, go to Editor -> Editor Settings, and to the Shortcuts tab. You can search for "Duplicate" and "Below" to find the corresponding commands and assign a shortcut to them.
OK it is M in my Godot
Seems it's not set to the default value. I've also had some issues in some versions with the shortcut not working. Shortcuts with Alt don't work well on Linux, like the one to fold code.