Click Start > Control Panel > Appearance and Personalization > Display > Screen Resolution or Right click on your desktop and select screen resolution. Select your main display and in the multiple displays drop down menu select duplicate these displays.

You can also press the Windows key on your keyboard + P and select the desired option to extend or duplicate the display.

Mac:

1. Make sure your additional display is properly connected and powered on
2. From the Apple menu, choose System Preferences.
3. From the View menu, choose Displays.
4. Click the Arrangement tab.
5. Enable (check) "Mirror Displays" in the bottom-left corner.