Yes exactly, birds of a feather fail together... an A380 has three primary flight control computers, but still carries another entirely dissimilar set of three flight control computers as backup.

Well, the diversity would cover the issue with random HW failures, not the case your SW has a bug in it. As to the SW, they _sometimes_ have vendor diversity.

Regardless, there are multiple fronts you need to tackle to have high reliability so you should use all techniques at your disposal.