2001 Tacoma 3.4l Check Engine

During the past year or 25k miles I've had problems with an engine
light with code p0420 which is cat efficiency low. I was told to
replace the secondary o2 sensor and clean the mass airflow sensor. I
replaced them both while I was doing it. That seemed to take care of
the problem until about a week ago when the same code fired. I always
check with the local dealer to see if there are others with similar
issues, but they're never much help. I am not against replacing the
dual cat system, but want some advise before I drop any more money at
this. Any help would be appreciated

Grif