This is working properly. Each Guinsoo's Rageblade gives you 1 stack per attack or spellcast. If you have 2 you get 2 stacks per, 3 for 3 ect. The max number of stacks is still 8.
The Black Cleaver works the same way, it does not increase the max stacks of the debuff but provides 2 stacks per physical damage source.
I'm aware that it's working as Riot intends it to, but the description is very misleading. The second sentence should really be UNIQUE. Same goes for BC I suppose.
It doesn't say UNIQUE because the effect IS doubling.
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