I have Niambi, Esteemed Speaker on the battlefield, nothing else, and another copy of Niambi in hand. My opponent tries to remove her with an instant spell. What happens if I cast the Niambi that I have in hand and it resolves? Can the new one bounces the target of the removal in my hand? Or the legendary rule triggers before her ability resolves?
The legend rule removes all but one of your Niambis before the ETB trigger can even make it to the stack, since the legend rule is a state based action, which are checked right after the Niambi spell resolves. You can't even target the one already on the field, since that is part of putting the trigger on the stack, if you choose to keep the new one that is. If you keep the old one, then the trigger can target it and return it to hand before your opponent's removal resolves, but you won't have the new one anymore.
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