Putting sensors around the loop would give confusing readings. You also mention use of a magic wand "Software programming to tell ..". In engineering, all magic wands work against you.
The way a safety engineer solves this problem is to start with a proper study of how sensors fail. We did a 5 year study.
There are only 2 fault modes, out of 14 fault modes that O2 sensors have, that causes them to read high. Both of those two modes are very rare, and both can be avoided completely by using the right sensors.
There are some modes that make sensors change slowly, with tendancy to low. All other modes simply cause the sensor to read low. By "low" or "high" we refer to a reading which is lower or higher than that expected after calibration. For example, a Sensor A reads 10mv in air, and Sensor B reads 8mV. In a PPO2 of 0.42, the first sensor reads 19mV so is reading low, but Sensor B reads 16mV which is reading neutral.
This means that if you have calibrated the sensors correctly, then if you have n sensors, the one that gives the correct reading, is the one that reads highest. The only time one has to do another check is if the PPO2 is reported to be falling, such as on an ascent, check that the sensor you are using with the high reading, reacts to O2 injection as fast as the others.
Once you can pick out which sensor is working out of n, adding sensors in one place, can give you whatever reliability you want.
Cheers
Alex