Sunhome, Fortress of the Legion

• WWizards of the Coast2005-10-01
  Official Ruling
  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.
• WWizards of the Coast2005-10-01
  Official Ruling
  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.

Rulings via Scryfall