Edit: Actually, hand-held items present the most problems in terms of bonuses. Binoculars and such are about the only ones that make sense, and they need special treatment anyway. Right now, if you are holding a pistol in one hand and a sniper scope in another, it has the same effect as if the scope was attached to the pistol! Ditto for the laser scope.

You could very well make NVGs, for instance, attachable to your vest or leggings.

Right now, if you manage to attach both the sniper scope and the reflex scope to your gun, their bonus will be cumulative

One solution is to make sure that you never have more than one such item active at any given moment. Another is to change the code so that even if you have more than one, only one will provide a benefit at any given moment. Which should it be?