Why would same diagnostic code up up twice for oxygen sensor that was changed and replaced already?



Presuming you didn't manually reset the code reader, which is done by using a computer plugged into the system (in which case it better be cleared for at least a short while or you've got a bad sensor, reader or elec connection), not terribly plain English of how I understand it:

There is a part of the sensor/computer operations that continually runs tests automatically and records values (that can differ), but if accumulated (the term I always heard) over a few runs indicate a problem then identifies and sends the error code. If you haven't reset it, just driving the car with a good 02 sensor through a few cycles...which are also determined by on off of key, until it overrides itself and sees a good reading and turns off light is all that is needed. Differs as does the computer determing good values and what tests to run when, but a handlful of on offs and 50 miles or so is standard to do it.