How dissimilar are redundant flight control computers? Facts

On Airbus aircraft there are computers to secure the flight envelope, or to move the control surfaces. FADECs totally control the engines. Computers take decisions in place of the pilots, or even against their commands. Boeing aircraft have similar computers, even if the crew has more authority. 


A330 electronic bay, source, photo by 'swiss_a320'

Safety implications

Being critical, systems are redundant and supervise each other to detect possible failures and isolate failed components. However a computer can still be developed from faulty specifications, or be wrongly manufactured, and a program can contains bugs. If the same defect is present on all computers on the production line, the purpose of redundancy may be defeated, as they wouldn't be able to detect erroneous behavior.

This is better stated in this article:


  Because of the severe consequences resulting from a single point of
  failure, hardware redundancy is critical in DAL A systems. But if the
  aircraft uses a redundant architecture built with similar channels,
  that system will still be susceptible to common mode failures that can
  cause all channels to fail in the same way.


Question

What are the principles used in aviation to reduce the possibility for redundant computers to fail or to make the same errors at the same time?
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).