Our Code Editor is based on Code Mirror and shares the same hotkeys. We only document the most useful and frequently used hotkeys but you can look at Code Mirror's list (link in our documentation) if you would like to see what else you can do.
It's also worth bearing in mind that other editors have different functions binded to Ctrl + D (sometimes to delete line in Eclipse for example).
That said, duplicating a line shortcut is a valid feature request and I can see places where that would be useful.
At the moment, you can Ctrl + C to copy the line (if you have nothing selected) and then Ctrl + V to duplicate the line.