A unit is only required to make a single Panic test during any Shooting phase. For example, if a unit loses more than a quarter of its number to shooting from one enemy unit but passes its Panic test, it will not have to take another Panic test during that same Shooting phase even if it then suffers more casualties from another enemy unit. This is because, whilst shooting is resolved one unit at a time, in reality all units would shoot more or less simultaneously. Therefore, all casualties would fall at roughly the same time.