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{T}: Add {C}.
{2}{R}{W}, {T}: Target creature gains double strike until end of turn.
If you give double strike to a creature after either first-strike or normal combat damage has been dealt, it won't help that creature deal any additional combat damage.
If you give double strike to a creature with first strike after first-strike combat damage has been dealt, that creature will also deal damage during the normal combat damage step.
