The Power Herb is a consumable item, which means it will disappear on use. Thus it cannot be used twice in the same battle or in the in-game playthrough. During "competitive" battles (post-game battle facilities and online battles), consumable items are only consumed for the duration of the battle and will come back once the match ends.
Edit: the move Recycle will restore the user's held item if consumed. Please bear in mind that this is generally not a viable strategy.
