Standard brief in my old company: "Confirmed by two or more parameters"
That is all well and good if you suspect a failure. But in this case no one suspected that the left attitude display had failed.
The attitude miscompute flags on the PFD on the CRJs are much less prominent than I would prefer. They are small amber boxes around amber text a bit offset from the centre of the PFDs. I'd prefer much larger text, right in the centre of the attitude display.