What is double strike?Edit
A creature with double strike deals damage during the first damage step at the same time as creatures with first strike. It also deals damage during the second damage step at the same time as normal creatures.
An unblocked 1/1 creature with double strike will deal 1 damage to the defending player and then another 1 damage for a total of 2 damage.
A blocked 1/1 creature with double strike will deal 1 damage to the blocker and then another 1 damage. If the blocking creature doesn't have first strike or double strike, it will only deal damage to the doublestriker if it survives the first blow.
A creature with double strike still attacks only once. However, an ability that triggers whenever a creature deals damage will trigger twice.
If all the blocking creatures are killed in the first damage step, the creature with double strike won't deal damage during the second damage step.