The entire item (BC) should be unique. And it should only apply the debuff to 1 target at a time. Instead of just blindly throwing your AoE around making you AND your ADC deal true damage to every single target.
Atm stacking armor is fucking pointless, stack HP and be sure to have a lot of CC on your team, otherwise, game over.
You'd think Riot would have anticipated this with all their testing. Pffft. Incompetent fuckups. Completely unacceptable to make this big of a mistake. This is something I'd expect from Blizzard to pull off, but not Riot.
Oh, and please Riot, make Randuin's Omen USEFUL for tanks to get. Right now, only bruisers will benefit from it (Oh, right, this is the bruiser patch, I forgot).