In dom2 you can wish for a unique not by his name but for his type. For example demonlord instead of the unique name. You get multiple demonlords of the same type but never the "real unique" one. But not sure how it works in dom3.
It is changed in dom3 - now you cannot have multiple unique units. So if you wish for "lord of the plague wind", then pazuzu turns up. If someone had pazuzu already he turns up "hostile" in the wishing province.