Dueling can be just as much a part of social and RP interaction as chat and emotes, so I don't think it should be automatically excluded from the social areas. On the other hand, it can get really annoying to have duels raging around you if that's not your cup of tea.

With this in mind, I suggest that for the areas that are being considered for non-dueling status, there be two types of instances, a duels allowed type and a no-duels-allowed type. When you choose your instance before zoning, you'd just select a dueling instance or a non-dueling instance--Whichever floats your boat. Ideally you could check a persistent box to show you only your preferred type of instance.

maybe put a time limit on how often you can duel in social zones might solve this issue?

One problem with that is that if two people are just trying to annoy others, they could stretch out a single duel as long as they'd like.