The resolution options offered in gnome-control-center's "display"
panel are too limited:

In particular for a monitor supporting up to 1920x1200, the resolution
option 1920x1080 is *not* offered. However it is useful if there is a
second display (TV) which should be configured in mirror mode and
where the second display only supports resolutions up to 1920x1080.

There seems to be no way to configure such a mirror setup via the
GNOME GUI. Note that I cannot configure the 1920x1200 monitor as a
mirror of the 1920x1080 one either: in that case only 1600x1200 is
offered as the resolution of the 1920x1200 monitor.

Ansgar