Was testing out black cleaver on kindred. When just equipped with cleaver my autos apply 2 stacks for some reason.
If I wield black cleaver and bloodrazor each auto only applies one stack on one auto
If I wield black cleaver and runaans each auto only applies one stack on one auto
However black cleaver + blade of the ruined king applies 2 stacks on one auto
Black cleaver by itself applies 2 stacks on one auto
TheLastShadow452 votesKindreds passive is a separate proc of damage. So 
will see both and apply two stacks. This could be why.
Cocho1 votesIts your auto + the % health passive
Hexs Fortune1 votesIts your auto + the % health passive
On hit effects don't usually trigger stacks. Also the stacks per auto varies based on items equipped
Cocho1 votesOn its just physical damage dealt. I bet a 
will make trigger 3 times in one auto with 
Hexs Fortune1 votesOn
Test it yourself. It only triggers once per auto attack if you have botrk and black cleaver, but it triggers twice per auto attack if you only have a cleaver