Lethality +15 = Unique Passive

It only doesn't stack if it's named.

Example - all stack with each other because the Lethality passives are not named. But three do not stack with each other.

both have named unique passives called Cleave, so you cannot have them both.

UNNAMED Unique passives stack if they're different items Unique passives WITH NAMES don't stack at all

It will stack with Lethality from other sources, but not with Lethality from other copies of the same item.

unique passives with names no not stack unique passive w/o names that are the same item do not stack. unique passive with the same name on different items do stack

kinda weird huh lol

Yes, if you build multiple of that same item then it won't give you more lethality