In windows XP you would right click on background, properties, settings, click on second screen. How do you do that for an Apple with HDMI?
2This is a CPU. I think you're misusing this term, because it's not related to your problem. Also, please clarify what you mean by "duplex". Do you want to mirror your screen to second display?– gronostajMay 16, 2018 at 8:12
Duplex means that information goes in both directions, on the same cable, simultanously. For example, ethernet cables are (today) duplex, because the same network card can both receive and send data on them, simultanously. In the case of monitor cables, it is not an important feature, the overwhelming majority of the data send on them goes from your computer to the monitor. (There is a very little information also to the other direction, this is with what your computer can learn, what is the wanted resolution of your monitor.)– peterhJul 13, 2019 at 8:00
However, on your question is it clearly visible that you are asking about dual-screen displays.– peterhJul 13, 2019 at 8:01
I'm not quite sure what you mean by 'duplex'. It's not a term normally applied to dual-screen setups.
Do you want to use the 2nd screen as a mirror of the first or a separate display?
You can do either from System Prefs > Displays > Arrangement