Multiple copies of Nadu's ability don't have independent trigger limits

[Grath]

Per MH3 Release Notes, each copy of Nadu's ability needs to count the trigger limit independently (also applicable if you flicker Nadu to refresh the ability):

If you somehow control more than one Nadu, each permanent you control will have that many instances of the granted ability. These abilities are not redundant. For example, if you control two Nadus and one becomes the target of a spell or ability, each of its granted abilities will trigger. If that same Nadu becomes the target of a spell or ability again later in the turn, each of its granted abilities will trigger again, as each of them have triggered only once so far this turn.

If Nadu leaves the battlefield and returns to the battlefield in the same turn, or if Nadu leaves and another Nadu appears, the triggered ability granted by the new one is different than the triggered ability granted by the old one. The one granted by the new Nadu may trigger twice for each creature you control, even if the ability granted by the old Nadu already triggered twice for those creatures that turn. (Note that creatures you control won't have the ability granted by the old one anymore, and if you control multiple Nadus, you won't have the opportunity to take actions before the "legend rule" gets you down to one Nadu.)