Should definitely be a bannable offense.
This and anything exploiting the duel ladder should be fixed or punished.
The problem is proving it is done on purpose. A disconnection can happen without being on purpose, even if the timing seems obviously to be going in favor of a on-purpose disconnect. You would need lots of accusating screens and probably videos to prove a fact like that.
A easy fix would be what you said :
*Devs can make it so you lose when you leave the server while in a duel giving points to the winner
But like Osiris also said, this is not being to be taken care of even if it sounds like it would be fixed in a heartbeat. You need to get in the ears of the devs and even then its not sure they will just go out of their way for you or anyone for that matter.