We had some word from Jesse Sky on this a while back (post here). He mentioned that dueling is disabled on the Fleets right now due to potential performance issues. Adding a specific Fleet dueling area may be possible in the future (and it's a great suggestion!), but it's not in our immediate plans.
Considering your game engine is a joke, and a full fleet creates enough latency as is, it's really a bad suggestion.
Those who PvP, understand this. The better alternative would be to use a Legacy unlock to shuttle to/from a planet for such combat, and far away from the Fleet as possible.