704.5k - If two or more legendary permanents with the same name are on the battlefield, all are put into their owners' graveyards. This is called the "legend rule." If only one of those permanents is legendary, this rule doesn't apply. [CompRules 2009/07/08]
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actually..it's one legendary card of the same name. I can only have one Niv-Mizzet, etc. out at a time.