The player shouldnt get hit / it shoulnt repeat

I want a "shockwave" to release when the player gets hit, but right now, the player also gets damage from the shockwave and the ability will just keep repeating over and over again as long as the player is missing 1 HP. Please help make it stop doing damage to the player and make it only blast once when getting the player is damaged, then after the cooldown it does it again when (and only when) the player is damaged damaged.