duplicate zoneMobSet additions fixed

src/engine/objects/Mob.java

@@ -806,6 +806,8 @@ public class Mob extends AbstractIntelligenceAgent {
         mob.npcOwner = guardCaptain;
         mob.spawnTime = (int)(-2.500 * guardCaptain.building.getRank() + 22.5) * 60;
         mob.BehaviourType = Enum.MobBehaviourType.GuardMinion;
+        mob.parentZone = parent;
+        parent.zoneMobSet.add(mob);
         return mob;
     }