The effect is unique and you can apply it only once per time else it will be considered too overpowered.
Remember when looking at items. Any item that has in parenthesis (Unique) and puts a name to that persisting ability on said item, it cannot stack. For instance Cinderhulk, the jungling tank item, and Sunfire Cape. Both very different items and built from different items, but they have the same "burning aura" effect on nearby enemies. Even though they deal different amounts of damage and are different items entirely, the burning aura effect says unique and has the exact same name as the burning aura effect from the other said item. Therefore, the aura effect does not stack. Same as building a Ghostblade, then building a Brutalizer. The armor-pen on both items have the same "unique" name in them and will not add to your armor-penetration.
Like they said unique passive, you never want to stack IEs unless you're being silly. Grab an IE and a BT or grab an IE and a Shiv or PD if you're looking for more crit strike in your build. But if you just want to troll someone you can grab as many IEs as you want.
Every item which says UNIQUE passive means that it's passive effect does not stack up. If the word UNIQUE is not written then yes, the effects do stack up.
passives would be brokenly overpowered if they stacked. The first example of this, was the old old SUNFIRE CAPE Evelynn build. They'd just buy five capes and go invis and burn people to death with her old invis mechanic. I used to play Zilean/Eve bot troll lane - sunfire capes for eve and I just speed here up so she could keep up with the ADC/SUPPORT and put bombs on her. I thought the enemies would see "floating bombs" but I learned since she was invis, the effect on her was invisible as well. Needless to say Riot got wind and fixed it fast. Still reminds me of the old Garen Comic:
I remember when they made the passive for Black Cleaver not unique anymore and everyone started stacking that item. League of Cleavers anyone?
Talon perma pick/ban. Even AD Carries like Graves were stacking them. I remember that EVERYONE was building 6 black cleavers, even supports sometimes. Those were fun times
Lmao could you imagine if they did? Just throwing out 300% crits like it's nothing, say good bye to your tank!
Like that doesn't happen alreayd nowadays with Vayne and Kog'Maw... And that damn Vayne is too popular right now for my taste.
No, and this is mainly done to prevent abuse from other players. Just imagine those crits stacking up... you could do 1v5 without any kind of problem.