Bergerie1 - All Boeing commercial transports have been fitted with two AOA vanes. More recent FBW models have used both vanes in conjunction with an AOA estimate to perform signal selection / fault detection to arrive at a selected and validated AOA signal to be used by the control laws. Measuring AOA in the traditional means using a vane that sticks out from the fuselage no more than a few inches is complicated by the local flow distortion. One degree of change in airplane AOA results in close to two degrees of change to the flow as measured by these vanes. Boeing has chosen to fit its airplanes with a pair of vanes mounted on symmetrically identical locations so that they are equally impacted by symmetric flow over the airplane. Vane location selection has also taken into account the distortion caused by sideslip trying to choose positions that minimize this.
As for 737 signal selection logic the historic approach has been to have the right flight computer use right side sensors while the left uses those from the left. The approach has been to flag differences between left and right with reliance on the crew to compare their flight deck instrumentation readings to sort out which is to be trusted. We will have to see if control law logic changes that are under consideration by Boeing include cross checking between the two AOA vanes to make the MCAS logic more robust to what appears to have been a single signal failure.