The only similarity is that they both dual wield.
Katar has a unique combo system, with a special bar that can upgrade the Katar's skills. Katar is also AoE based and has a lot of self buffs/heals.
Duelist is all about evasions and hitting people with all their DoT's. It doesn't have very many self buffs or heals, rather poisons and crit boosting skills.