This is the exact same ammount of xp, only it comes up twice. Sorta like the same thing with loot, sometimes you kill a single enemy but he drops double loot.
i think it happens when you have a weapon that has life drain/lightening bonus that can do extra dmg on top of your original dmg. if say your enemy has 3 health left and you hit him for 30 and your life drain procs for 4 dmg, he die twice to your two simultaneous attacks.
this happens a lot when i use a dagger that has lifedrain(i think, or lightening or something). and it is really sweet when it's a boss that gives tons of exp. although for those bosses, saving/loading is almost mandatory so even if i do get a double-exp proc, i probably will need to load anyways.
