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.
01-10-2005
wotc
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.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.