Clarification - When you say duplex do you mean a breaker with 2 double pole breakers in one or are you talking about tandem breakers?
Anyway, to the best of my knowlege GE does not make a duplex or tandem breaker, instead most of their panels accept half space breakers (THQP series) which can be connect together to make double pole breakers. They do have THQP double pole breakers. I may be wrong or not even answering your question, but that's the best of my knowlege, hope it helps.
[This message has been edited by Electric Eagle (edited 09-11-2002).]