Mirroring displays in order for the same image to display each computer and external display or projector.

Mirroring displays the same image on each display.

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. In the example below. the blue boxes represent each display that is connected. The white bar at the top of both boxes represents your menu bar and is shown on both displays as they are showing the same information.

Your Mac will attempt to match the closest resolution between the primary (or built-in) and secondary displays.

The usable resolution is the closest matching resolution for both primary and secondary display. If you choose a resolution that is not supported on both displays, the resolution will return to the closest matching resolution.

Reference: How to use multiple displays with your Mac Learn: how to use multiple displays with your Mac, with video mirroring or extended desktop.