As in the title - if I have an item, for example 6 helmets for a paratrooper squad, I should be able to equip the same 6 helmets for all 4 campaigns, not buy 24 of them:
EDIT: The same should apply to the vehicle decals and decorators - a decal should be bought once and then become free to use on all tanks at the same time the way it is in War Thunder. Or at the very least buying a decal X amount of times should allow you to use it X amount of times on the same vehicle: you get one decal from the current event, you can put it once on every US tank you have. You get two, you can put it twice on every US tank.