702.4bIf at least one attacking or blocking creature has first strike (see rule 702.7) or double strike
as the combat damage step begins, the only creatures that assign combat damage in that step are
those with first strike or double strike. After that step, instead of proceeding to the end of
combat step, the phase gets a second combat damage step. The only creatures that assign combat
damage in that step are the remaining attackers and blockers that didn’t have first strike as the
first combat damage step began, as well as the remaining attackers and blockers that currently
have double strike. After that step, the phase proceeds to the end of combat step.