It's also important to have diversity. For example, on the Boeing 757 there are two computers that control the stab trim, that do the same thing. They must agree or both computers are automatically locked out.

The two computers are developed by two independent teams who are not allowed to talk to each other. Two different CPUs, two different algorithms, two different programming languages.

The idea, of course, is a design problem with one does not propagate to the other.

If one program is used in all your infrastructure, all your infrastructure is compromised when that program has a bug.