Like the 20% cooldown from
and 
or the hsp % increase on
and 
The lethality passives do stack due to how_**UNIQUE **_ passives work. Unique passives do not stack from multiple of the same item. You cannot buy 3 duskblades to get 3x the leathality. You can buy the 3 leathality items to do the same thing though.
Compare this to the Unique Named passives. For instance: spell blade from
and the items it builds into, do not stack no matter what item you buy. If you get a
and a
you only get 45 ms as the "enhannced movement" passives do not stack.
If it reads: Unique passive
then two of the same item will not stack.
If it reads : Unique passive - (a name here, ie "spell blade" or "enhanced movement")
then you can only gain the effect from one item.