You can use xrandr to duplicate the image.
Common display names are:
LVDS - Laptop Screen
HDMI - High-Definition Multimedia Interface
DVI - Digital Visual Interface
DP - DisplayPort
VGA - Video Graphics Array
TV - Composite Video
S-video - S-Video
run xrandr from terminal to see what your displays are:
~ $ xrandr
Screen 0: minimum 8 x 8, current 1440 x 1024, maximum 8192 x 8192
VGA-0 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 340mm x 270mm
1280x1024 75.02+ 60.02
1024x768 75.03 70.07 60.00
800x600 75.00 72.19 60.32 56.25
640x480 75.00 72.81 59.94
LVDS-0 connected primary 1440x900+0+0 (normal left inverted right x axis y axis) 367mm x 230mm
1440x900 59.94+
HDMI-0 disconnected (normal left inverted right x axis y axis)
Once you know what your display names are you can use xrandr to duplicate/mirror your display.
In my example to mirror/duplicate/clone the displays I used: (laptop with a monitor connected via VGA)
xrandr --output LVDS-0 --auto --output VGA-0 --auto --same-as LVDS-0
Once you happy with it you can make it permanent by adding it ~/.xinitrc
https://github.com/recalbox/recalbox-os/wiki/Change-display-resolution-on-x86---x86_64
