How dissimilar are redundant flight control computers?
As far as Airbus is concerned:


Each unit is composed of two dissimilar boards, one driving the output and the other checking it. Dissimilar means both different CPUs and chipsets (A320 uses i386 (Intel) and m68k (Motorola); newer models use different combinations, basically whatever was widely used at the time they were designed) and software written by two independent teams.
There are fail-overs, two or three depending on the system (IIRC the unit reading the side-sticks is the only one with four copies).
The two main axes, pitch and roll, are controlled by two different systems. ELAC controls elevator and ailerons, SEC controls horizontal stabilizer and spoilers. This is two completely independent chains including different actual control surfaces except for the side-sticks.
A320 has (hydro)mechanical backup for pitch via the trim wheel and yaw via the pedals, utilizing yaw-roll coupling for roll. This works even with complete electrical failure. IIRC the backups on newer models don't though (because complete electrical failure has never happened).