Pretty much. It *is* possible for the same unique to drop twice in the same game, but only if it drops at the same time on one monster (i.e., mephy drops two Chance Guards or something). Once a unique has dropped though, that same one is out for the rest of the game (other uniques can drop, just not that one).
The double-unique drop is really just a curiosity though. The chances of it actually happening are even less than those of you finding two legit Zods in your lifetime
