I think restricted attachments should also have a rule that no duplicate copies of that restricted attachment be on same hero/character. Restricted attachments says only two can be attached on a hero/character. No hero should have two Citadel Plates (restricted item) attachments on him (and get +8 hitpoints). Same thing with 2 Blades of Gondolin or 2 Dwarven Axes on 1 hero (though possible with 2 hands, Blades' response triggers of putting progress token also duplicated w/c is kind of weird).

If your issue is with the fact that it doesn't make sense to be wearing 2 sets of armor or something, then I believe it even says in the rulebook something along the lines of thinking of it as having 1 set of improved armor rather than 2 different sets.

Otherwise, they can simply make an attachment unique if they don't want multiple copies in play.