@GeneralHandGrenade I disagree.
Unlike A&A, I find no mention in the AA section that “each attacking[/defending(in GW)] air unit may only be fired upon once.”
That leads me to believe the key phrase is “An AA gun”. While each AA gun can fire at up to three different air units, it can only fire at the each air unit once. That makes me think that two AA guns can fire at the same aircraft.
e.g. two fighters are attacking, two AA guns defending. Both guns have two shots at each aircraft, for a total of four dice.
Or if you have between 4-6 planes and 2 AA, you roll six dice.
The Coastal Artillery rule is written in the same way.
If you have three tanks attacking one infantry, you wouldn’t roll one die for the tanks. You would roll three dice because they all can hit the infantry.
Anti-Aircraft: An anti-aircraft gun (AA) fires specifically at aircraft. It does not Defend facilities from strategic bombing (although facilities have inherent AA guns which function identically to a normal AA gun but do not move). An AA gun rolls one die for each opposing aircraft up to a maximum of three rolls on a hit of 3 or less. The player suffering the hit chooses which aircraft to remove as casualties and those aircraft make their Attack/Defense rolls as normal. AA guns fire on the first round of combat only. The gun may be taken as a casualty in any round. Artillery or AA can move two zones when paired 1:1 with motorized infantry.