Can't always trust animations! Duriel actually does 3 attacks when he uses Jab, even though it looks more like he does only 2.
And about his attacks:
If his target is outside of his reach (melee range), there's a 60% chance he uses Charge, and otherwise he'll walk towards his target.
If the target is within his reach:
There's a 16.5% chance for Attack 1, which is a regular attack that applies stun (puts the target into hit recovery).
There's a 16.5% chance for Attack 2, which is a regular attack.
There's a 33% chance for Jab.
There's a 34% chance for Smite, which always hits and applies knockback.
(Faster Hit Recovery will decrease the length of the stun from Smite, Charge and Attack 1.)
And a warning in advance.. his Charge on Hell deals 413-574 damage, and that has a 5% chance to crit for 826-1148.
On top of that, the only thing that can make Charge miss, is being out of range at the moment his swing "lands". Other than that, it is a guaranteed hit. (And I do mean guaranteed -- shield is ignored, Dodge is ignored, Defense Rating is ignored, etc. It's 100%.)