Fix runtime crash related to scrambling. (#11932)

game-app/game-core/src/main/java/games/strategy/triplea/delegate/battle/AirBattle.java

@@ -99,7 +99,9 @@ public void updateDefendingUnits() {
   @Override
   public Change addAttackChange(
       final Route route, final Collection<Unit> units, final Map<Unit, Set<Unit>> targets) {
-    attackingUnits.addAll(units);
+    // Avoid duplicates. Note: This is needed as scrambling code calls addAirBattle(), but the
+    // battle may have already been created with the same attackers. They should not be added twice.
+    units.stream().filter(not(attackingUnits::contains)).forEach(attackingUnits::add);
     return ChangeFactory.EMPTY_CHANGE;
   }