Due to a bug, my dual-monitor system booted up with the desktop
duplicated on both monitors. When I opened the Display Configuration
to correct this, it showed only a single monitor (DVI-0). This led me
to initially believe that the monitor detection was flawed. However,
I accidentally dragged the DVI-0 box and discovered that DVI-1 was
underneath it. Once I thus realized there were two boxes, I was able
to put them back side-by-side and restore the setting I desired.
The system needs a better way to convey the fact that the desktop
is duplicated across multiple monitors.