Consider the following scenario:
On a Windows 7 laptop with hybrid graphics,
Connect multiple monitors to the system, including 2 DVI or DisplayPort monitors
Configure duplicate (clone) displays between the DVI or DisplayPort monitors.
Set the duplicated external monitors to be the primary display.
Restart the system.
You may observe that the primary display has been changed to the laptop screen instead of the external monitors.
With multiple graphics adapters and some monitors that are configured for duplicate (clone) mode, the monitor configuration may be saved incorrectly to the registry. As a result, when Windows attempts to restore the display configuration on the next boot, it fails and the configuration previously set may not get restored properly.
To work around this issue, manually reset the primary display configuration after restarting the machine.
for other considerations.