Class moderation meaning each class only fights their own class. BM vs BM, Barb vs Barb, Wiz vs Wiz, etc.
The dueling is a nice idea for some classes, like barbs, since two equal geared barbs can pretty much never kill each other in PK unless one gets a string of like 10 zerks. But at the same time, it can be pretty bad for venos, since, like Elly said, the first to stun (nova) will win generally.
Also, cleric vs cleric would never end. You'd have to limit or completely stop them from using heals.