Double attack, or as phrased within the game strikes twice or shoots twice, is a special ability of some creatures, that enables them to attack the same target twice in one combat round. Creatures with double melee attacks execute their second attack right after the attacked stack has retaliated. Shooters who shoot twice do not get the second attack if engaged in a melee fight.
Creatures with double attack:
Ayssids have a Ferocity ability which means a double attack if any unit in the attacked stack was killed by the first hit.