Soo I was in practice mode messing around and got a black cleaver on Ezreal, I noticed his Q started applying 2 stacks of black cleaver every time it hit.
My build was:
Blade of the Ruined King
Boots of Swiftness
Duskblade of Drakthar
Lord Dominiks Regards
In that order.
I tried other items, and if I swapped around the order of Blade and Tri-force it would only give 1 stack?
It was a weird interaction, not entirely sure if bug or not?
I have recordings of it if needed.
Also works on champions, tested on the bots.