As a NIN player myself, I was extremely confused by the tooltip as well.

Definition of "Doppleganger" (as ripped from Dictionary.com): "a ghostly double or counterpart of a living person"

I did not understand that my enmity was moving to whomever I used it on. I got the impression that I was creating some sort of NPC "shade tank" or something of the sort. After repeated uses, it became clear, but the tooltip is extremely misleading.