98 Blazer o2 sensor

My 98 blazer is going through o2 sensors. It's the one after the cat. converter, I've got it done two times and the check engine light is back on again.
There was a tuneup done on it, and the catalitic converter was also replaced. It seems the mechanic doesn't realy know what the real problem is. Can anyone help me out?

Check your air fuel mixture, if there is too much getting burned the 02 sensor can get torched and not work properly.