Vipilot
Having been involved from the flight ops side, we argued endlessly with the bean counters to quip with dual FMS. Our argument is that the landscape in technology terms has about a 10-15 yr turnover, so if for nothing else for the future issues that may not be aware of, we need to spend the extra cash.
Turns out we were both right. We made it a a couple of decades on a single FMS but now are paying the price with diminished capability with NAVAID outages and pending removal. Nothing like having to cancel because the VOR at a small field is down and there's an overcast layer a couple thousand ft high that otherwise wouldn't slow you down.