Don't discourage them to remove double-res tank items. It's bad enough that you're resigned to a boot slot and Merc isn't helping anyone when knockups are so proliferated.
It's there for 'all forms of damage' because there's a ton of damage and damage itemization is really looking into mitigation these days. Meanwhile, these items aren't just resistance dumps; they have actives that aid tanks in doing their job, because a lot of them needed the help and penetration is at an all-time crest for AD, between Mortal and Dom's.
It sounds like you want them to address stacking more than the item itself, with which I could wholly agree. 25% value for stacked shields seems fine-- why are you even trying to layer two to begin with? That clearly shouldn't be an intended function of the item. Meanwhile, being able to put the shield up to about 400 means that added shields will be extra 100 absorb, so that's both helpful if you're really determined and not imposing enough to just outright deny all the damage.
Does Locket itself really need to be taken away from?