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

THERE IS MORE THAN ONE

Answer

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.