Check the name of the active displays.

xrandr

Sampleoutput:

• eDP-1 connected primary … → primary monitor
• HDMI-1 connected … → secondary monitor

More Infos

xrandr --output HDMI-1 --same-as eDPA-1

Replace the display names acording to your system.

Add the xrandr … command to the 'Startup Applications'

More infos.