In the Blessed and the Damned, Legendary Pokémon are given auras that grant special abilities to make them even more formidable. Three of these allow the user to summon a combatant of a level equal to their own (Or half their level if the combatant is a trainer).
I can not find any limitations to these combatants other than the level. Could this aura be used to summon an exact duplicate of the Legendary in question? I understand that in many cases this makes no sense, but it could be a way to explain Celebi’s gimmick where dozens of it from different times appear: One summons one other. Next turn, both summon one more each- Etcetera.