Doppelganger

 This card's Japanese name is a pun of Doppelganger, since there is already a card whose Japanese name is Doppelganger (Greenkappa).

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A similarly-named card had been used in the Yu-Gi-Oh! (Duel Monsters) anime series before this card; for that card's TCG release, its name was changed to Mimicat.

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OCG Rulings

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The activation of the card "Doppelganger" can be performed at any time, except during the Damage Step.: Doppelganger

The effect "When you take damage from the effect of a monster your opponent controls, inflict the same amount of damage to your opponent" makes a Chain Link. (It does not target.)

The effect "inflict the same amount of damage to your opponent" always activates when the activated effect of a monster in your opponent's Monster Zone inflicts damage to you while you control "Doppelganger". (It activates even during the Damage Step.)

The effect "inflict the same amount of damage to your opponent" can be activated in the same Chain Link as the card "Doppelganger", except during the Damage Step

Previously Official Rulings

"Doppelganger's" effect starts a Chain after the effect that inflicted damage resolves.

If you take damage from two or more effects in the same Chain, "Doppelganger's" effect will activate once for each of those effects after the Chain resolves. Then, "Doppelganger's" effects will form a new Chain.

You cannot activate "Doppelganger" (flip it face-up) during the Damage Step.

"Doppelganger's" effect will activate during the Damage Step if it was face-up on the field.

If "Cannon Soldier", etc. is Tributed as a cost to activate its own effect to inflict damage, "Doppelganger's" effect will still activate.

If you take damage from multiple monsters' Continuous Effects at the same time, like "Rapid-Fire Magician", "Doppelganger's" effect only activates once, and inflicts damage equal to the damage from only 1 of those monster's Continuous Effects.

You can activate (flip face-up) "Doppelganger", and activate its effect as part of the same Chain Link, after a monster's effect that inflicts damage to you resolves.