[naloth]

Offhand, I'd go with Affliction-Duplication linked to Mind Control if you can exert some control and get benefits other than it distracting an opponent (ask it questions, have it help you in other ways, etc). Creating a super minion is incredibly useful.

If the evil twin exists only to vex/annoy the person it's cast on, then enemy seems good enough. It would not have any use beyond occupying your opponent.