User blog comment:Hamengeri/Hamengeri's small item overhaul/@comment-5686533-20140916052815/@comment-24558666-20140920114356

They are not supposed to stack on same target. In this case, it would not make you pick either Rabadon or DFG, but would make you immune to DFG damage amplification if you had Rabadon as Rabadon applies Doom(possitive) onto you, while DFG applies Doom(negative) onto enemy.

TL;DR: If you want people to either pick rabadon or DFG, this is not right way.