The OP did have the after cat sensor replaced. Also, the code is not telling you to check the after cat sensor. It is telling you to check the cat!!!!!
Here is an analogy to help you understand how P0420 is generated: Think of the cat as a muffler, and the O2 sensors are microphones installed before and after the muffler. A microphone generates voltage as the magnet moves back and forth, and an O2 sensor generated voltage in the presence of oxygen. In the example, mike #1 is measuring sound, the sound gets attenuated in the muffler so the sound level coming out of the muffler should be quieter than what is going into the muffler. If this is the case, mike #2 should not pick up as much sound as mike #1. If mike #2 picks up the same sound as mike #1, the muffler isn't working and so P0420 is set. Jeff's assertion that if O2 sensor #2 is picking up a signal, it more likely to be bad than the cat being bad is like saying that if mike #2 is picking up sound, it is more likely that mike #2 is bad than the muffler being bad.
Here is another analogy along the same lines... a librarian who is deaf has to enforce a no-nose rule in a closed room with no windows. Since the librarian cannot hear, they install a microphone in the room and hook it up to a sound meter. By looking at the meter when the room is empty, the librarian knows the level of background noise when the nobody is making noise. If the librarian sees the sound meter moving, then using Jeff's logic, then the microphone must be bad because it is picking up sound. What is more likely? That the mike is bad because it is picking up sound or that there is noise in the room? Under the logic programmed into a car's computer, if sensor #2 is seeing activity, it will set P0420.
Since I posted previously that multiple re-occurrences of P0420 seems to be common in Corollas, there may be another explanation for the code and I recommended that the OP have it checked out at a dealership. Perhaps the threshold for turning on the MIL is too low?
Ray O (correct punctuation to reply)