Was just playing 
in a customs match, bought Youmuus and had 29/7%, then I bought Duskblade and had 44/7%. What am I missing? Lethality is a unique passive, so why does it stack? Or is it not a unique passive?
Its unique to the item. Building 2 Ghostblades doesn't give more lethality.
They're not named passives so they stack. 
and 
for example both have their passives named as "Lifeline" so they can't be stacked. The lethality items aren't named so they can stack. The reason why they say UNIQUE is so you can't build the same item twice like 2 
or some shit.
It just doesnt stack with itself since its not named
Unique passives without names will stack with other items. They just don't stack with the same item.
Unique unnamed passives stack between different items but not duplicates of the same item.
+ = 10 Lethality
+ 
= 25 Lethality
Thanks everyone, never knew that.
just like zhonya and stopwatch
