Rapidfire Cannon and Stakkik Shiv have a unique feature.
Compared to other items which share similar effects, they're the only two items that work in tandem with each other rather than overwrite the other.
As examples:
Do NOT all stack, as the spellblade effect is unique. (EDIT: People have corrected me in saying that while the damage doesn't stack, the AoE effects do)
Do NOT stack, but you're free to swap between the two.
Do NOT stack, even though the damage effect is different between champions and monsters.
Technically stack but you have to finish one tear before the other can begin stacking.
Do NOT stack unless on different champions.
Do NOT stack
DO stack, get the full effect form both and both charge under one energize effect. (It won't charge faster but it'll combine the two charges)
I know the list of champions who can use both is likely limited to, say, Vayne and Tristana but it's frustrating that these items don't have consistency. Can we make the energized effect unique so you have to decide between having long-range poke or wave-clear. If anything else, I'd just do it for the sake of consistence.
Well
We COULD make it so that all the other effects aren't unique...

Unique passive : Energized strike. (yes, even the component item.)

. Not even pros in lcs goes 100% pure on item build because hurdur i got a team to bail me out. the situation is that you get cc for once in late game and you dont have a qss but hurdur 600dmg rocket you will die instantly without doing shit
<-- those unique, yet identiacal, actives stack.

don't stack because they are virtually identical on top of sharing a named passive, the stronger effect takes priority, just like with Spellblade's unique damage modifier.
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i remember these days
was worse tbh, since she used to stay stealthed even when right next to you.