If Phantom Dancer is supposed to be a dueling item...
Unique Passive - Lament: Attacking an enemy champion causes them to deal 2% less damage to you. This effect stacks up to 5 times, and attacking a new target resets the stacks (stacks fade after 10 seconds of not attacking).
(This effect is applied like Vayne's Silver Bolts in regards to interactions with things like Runaan's.)
I mean, PD already gives the best raw stats of all the Zeal upgrades while being cheaper than them to boot. An instant 12% damage mitigation from whoever you attack is rather excessive on top of that.
Having to actually be dueling someone to get the full effect of the mitigation seems like a logical step to me. Plus, seeing as it gives the most Attack Speed of the Zeal upgrades anyway, it's already designed to hit things quickly in a short time anyway.
(Also, why are the two passives on PD named? There are no other items in the game that have either passive...)