Lethal Distraction
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Lethal Distraction • Shadows • Action • Rare • Play: Choose a creature. For the remainder of the turn, whenever this creature takes damage, it takes an additional 2. • Artist: Raphael Massarani • Card Number: AoA:305, WC:272
Official Rulings

The creature takes 5 damage total: 1 from the initial damage, 2 from the first Lethal Distraction, and 2 from the other Lethal Distraction.
Each Lethal Distraction's "additional" damage simply adds 2 to the original instance of damage; it is not its own, separate instance of
damage. Therefore the Lethal Distractions will not trigger off of each other's damage.Source: Rulebook
I play two copies of Lethal Distraction (AoA 305), choosing the same creature each time. Later in the turn, I deal 1 damage to that creature. How much total damage does the creature take?
The creature takes 5 damage total: 1 from the initial damage, 2 from the first Lethal Distraction, and 2 from the other Lethal Distraction.
Each Lethal Distraction's "additional" damage simply adds 2 to the original instance of damage; it is not its own, separate instance of damage. Therefore the Lethal Distractions will not trigger off of each other's damage.
Source: Rulebook
Commentary
If an ability says "Choose a creature," the Active Player may choose a Friendly or Enemy creature when resolving the effect.
Source: Rulebook