Question: Can you only have one instance of certain summons?

[Valerius]

Originally Posted by Anaconda

I understood Eater is an unique creature. Only one can exist at the time.

The eater of the dead has multiple forms. Its initial form is unit 994 and the game checks to see if that unit exists. If it does, then casting the spell again will fail. Once the eater grows in power and shifts to unit 995 then you can successfully cast the spell again since the game will not find that unit 994 already exists.