I been watching videos of ILS approaches and I can't seem to understand why on the Flight Mode Announcement of the PFD it says sometimes "cat 111 single" and sometimes "cat 111 dual".
I'm not a professional pilot but I can give you the gist. Generally airliners will have at least two autopilots, sometimes three, each with their own air data, inertial reference data inputs, different ILS recievers, difference power sources etc. The idea being that each autopilot is as independent as possible from the other.
Normally only one autopilot will be engaged, but during an autoland in low vis, all autopilots will be engaged to provide redundancy and error checking (look up fail passive/fail operational for more info).
The single/dual displayed on the PFD is indicating that one/two autopilots are in operation respectively (I'm guessing it was a 737 you were watching?).