A Replacement Effect can only be applied once to an event, or to any Game Actions or events that replace that event. Example: A player plays a spell that reads “gear you control become 1 [M] gear units this turn.” They control two copies of Zhonya’s Hourglass when the spell resolves. If one of those copies is killed, both of their Replacement Effects will be applied. Whichever is applied first, that Replacement Effect can’t be applied again. When it is applied, it kills its source, which creates an event the other can apply its Replacement Effect to. Once they’ve both applied their Replacement Effect to the original death event and the event that replaced it, they cannot go any further. At that point, whichever Zhonya’s Hourglass applied its Replacement Effect last will die.
§ 370.2 Core Rules PDF (officieel)
§ 370 A Replacement Effect can alter the typical flow of play, including other cards' executions.
