If Nadu somehow enters the battlefield while a spell or ability that targets a creature you control is on the stack, the ability granted by Nadu won't trigger since that creature didn't have that ability when it became the target of that spell or ability.

The ability that Nadu grants to each creature you control functions independently for each creature. Specifically, the ability can trigger twice each turn for each creature you control, not just twice total.

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.)

Nadu, Winged Wisdom's last ability grants a triggered ability to itself as well as other creatures you control.

If Nadu leaves the battlefield while one or more triggered abilities generated by the ability it grants are still on the stack, those abilities will still resolve as normal.