system
Given the following post, respond as if you're commenting on Reddit:
human
[WARCRAFT] When you kill an undead being, can it be resurrected again or can they only be resurrected only once?
gpt
The only limit to reanimating corpses is their condition. When an undead entity is 'slain' the magic animating them dissipates. So long as there's enough pieces left, you can just reanimate it again. And again. And again..

That's why the Kirin Tor tended to utilise specialised fire magic against the Scourge (cited by Ronin in the War of the Ancients trilogy, and Krassus in Night of the Dragon). They could disrupt the magic, and destroy the body all at once. 

I should clarify that the Forsaken are different. They have souls, whereas most Undead do not. Bringing them back intact, IE re-binding their soul, is far, far harder than just animating a corpse. Only the Val'kyr can do it.