[Actually it swaps between the captain's and the first officer's Angle of Attack sensor each flight.]
The FAA somehow certified a whole series of aircraft (B737MAX-8, -9, -7 & -10) with a new system ("MCAS") critical to flight safety which relies entirely on one out of two available sensors.
There is no averaging, no voting, not even a 'disagree' indicator.
The pilots' flight manuals originally omitted any mention of the MCAS, so if it misbehaved, the pilots would have no idea WTF was happening because they did not know MCAS existed.
Oh, and just to add insult to injury, having the AoA displayed on the pilots' screen was an 'extra cost option', so unless their airline had gone to that expense, there was no way for the pilots to know that their AoA devices were in violent disagreement
In my view the FAA acted completely negligently in certifying the B737MAX aircraft system (including the manuals). It seems likely that they relied on representations from Boeing that negligently (or even willfully) misrepresented the risks inherent in the MCAS design and implementation.
Since there were US citizens killed in the Ethiopian crash, if that is shown to also be MCAS-induced, I would expect both Boeing and the FAA to be hit with ginormous lawsuits in US courts.
Then there will be the teeny weeny detail of fixing and re-certifying "anti-death-crash software patches", manual updates and probably hardware changes - maybe including a third AoA transducer and voting / averaging / alerting software