You cannot double the effect of “Fairy Meteor Crush” by equipping two copies to the same monster.
If you equip your “Fairy Meteor Crush” to an opponent’s monster, and it attacks a Defense Position monster on your side of the field with ATK greater than your monster’s DEF, the opponent will take the difference as Battle Damage. This is because “your opponent” is the opponent of the player who controls “Fairy Meteor Crush” not necessarily the opponent of the player who controls the monster.
“Kuriboh” or “Waboku” can be used to prevent the damage caused by “Fairy Meteor Crush” because it is Battle Damage.