Handle mirrored monitors on X11

Handle mirrored monitors on X11

macOS and Windows present mirrored monitors as a single virtual monitor,
but X11 exposes this scenario as two distinct monitors with identical
coordinates. This messes up our logic, and is likely confusing for the
user. So instead we'll ignore any monitors that have identical
coordinates to any already seen monitors.

(cherry picked from commit fb561eb321b36dcf13932c903047ce08e55d67d5)