In the keep, while fighting multiple paladins, I noticed that one of them had three stacks of Zealous Focus.

 

Does it stack? If not, why show it three times?

It's probably displaying because, although the effects don't stack, the game displays such things regardless. My main character is a paladin, and while Pallegina was in my party we both had the Zealous Focus aura (she came with it ). I get two icons for the buff, but if you mouseover the buff list, one will be greyed out and say "Suppressed". 

 

I imagine it's the same for enemies, but  you can't mouseover their debuffs (to my knowledge).