As I say, it's probably better to make it GM Fiat, and that's what I'd recommend, but we're exploring. And maybe we'll end up with something that players can use.
I'll need to use Great Power to make this work; I have to do some fiddling with Extras and Limits.
We start with a description. The bad guy touches the hero with his Ebon Rod of Xorex and poof, a duplicate instantly springs up, except he or she is Eeeeevil, and ready to fight the hero to the Comics Code Approved unconsciousness. We don't need to do this an infinite number of times; four or five evil duplicates should be sufficient, and the bad guy can only duplicate a person only once per battle. (Or once per adventure; that sounds equally Silver Age.)
One possibility is Servant. The problem with Servant (as it always seems to be) is that the created servants have no mental abilities. You can't really make an evil duplicate, though you can make a robot duplicate. I could invent an extra for that, but I have wonky rules for myself for creating extras. For this kind of thing, I don't want to handwave it away and say, "Oh, there's an extra on that."
Power Mimicry might be another way to go: Power Mimicry, Extra: Affects Others, Limit: Extra Only. That turns someone already of evil mind into a copy of the hero's powers, but it doesn't quite feel like an evil duplicate. It's an evil mimic, sure, but it's not quite a duplicate. Plus you have to add on some other extras, like Mental Duplication, Specialty Duplication, Transformation (limited to their form). And to make sure it works, it has to be Power Mimicry 10.
Except that one of the terms of the power is that if you're knocked out, the mimicry stops. It doesn't sound like you can handle multiple people, unless you invent an extra to do that. I'm not comfortable with that extra, either. (Though for people like The Composite Superman or the Super-Skrull, it might be just the extra we want.)
Okay; so let's try the other obvious power: Duplication. If you want to duplicate four heroes, you need Duplication 4, Extra: Instant, Extra: Affects Others, Limit: Extra Only. Let's add another Extra/Limit pair: Extra: Multiple targets; Limit: Only one copy of each. That feels to me more like what we want. What about the evil self part of the description?
How about a Secondary Effect, either Mind Control or Emotion Control? It could be Emotion Control that only instils a hatred of the original person, or we could go for full-on Mind Control. (Or we make it GM's Fiat) So that would be Extra: Secondary Effect (Emotion Control), Limit: Only hatred of the original, Limit: Situational--Only duplicates created by the Ebon Rod of Xorex.
What if we add another modifier onto the Duplication, which somehow makes the duplicate vulnerable to mind control from the holder of the Ebon Rod? I'd probably make it an Extra, because it's an advantage to the wielder. (You don't need it: The duplicate is an NPC character, and if the GM wants it mind controlled, it's mind controlled, darn it.)
If we go Emotion Control, we leave open the interesting possibility that people who aren't the target can get through to the evil duplicate. Maybe that's a loved one or a teammate, but someone convinces the evil duplicate to trash the holder of the Ebon Rod.
With Mind Control, you don't have that, he's just evil. His Qualities might be in opposition to any good ones. The add-on looks like this: Extra: Secondary Effect (Mind Control), Limit: Situational--Only duplicates created by Ebon Rod of Xorex.
If you're mindful that players might want this strange brew, maybe you should add this to the Duplication: Extra: Duplicate susceptible to mind control from holder of the Ebon Rod. If only NPCs are going to use this mix, you don't need to. NPCs do whatever you as GM tell them to do. But if players are going to use it...
You know what? Mind Control or Emotion Control are just bogs to get trapped in. By analogy with the whole Marked Thrall limit, we're going to create an Extra that's only necessary because we threw Affects Others into the mix: Extra: Duplicate has opposite moral orientation than original.
Now the power looks like this, where X is the number of heroes we have to deal with.
Duplication X; Extra: Instant; Extra: Affects Others; Extra: Duplicate is moral opposite of target; Extra: Multiple targets; Limit: Only one duplicate per battle per person; Limit: Source (Ebon Rod of Xorex); Limit: Extras onlyThat's four extras and three limits, so a player could actually have the Ebon Rod of Xorex as a tool for the cost of two powers....
I'd also let a player spend a determination point or advantage to weaken the last limit; by using a strength or coordination maneuver, they could get the villain to touch himself with the Ebon Rod, and they'd have a good copy of the villain to fight on their side....
If you want, you can eliminate the Instant extra and in fact make it sloooow. (Maybe the duplicate is actually a clone grown in a vat somewhere.) Then the evil duplicate is let loose in society to wreak havoc on Our Hero's personal life. Which the duplicate knows all about, because it's comic book cloning.