I've been thinking about evil twins recently.
There are a few different ways to parse "evil twins" so let's unpack it a bit.
I mean duplicates of the characters who differ in a significant moral or methodological way. Powers can differ slightly, but really the focus is on morals or technique. So the folks who are thematically related but have different powers or the actual siblings who happen to be bad or the arch-foe who stands for everything the PC doesn't, well, those are outside of the scope of what I'm thinking. Evil Kirk is just barely inside the scope.
The Justice Lords from the Justice League animated series are probably the best example of this line of thought. There are others, of course: Dark Beast, Dark anyone
Really, this is an example of "If this goes on..." — the character represents something taken to a logical extreme, and (we hope) forces the player to think about what the character's actual limits are. You might have a Wolverine-style character who meets Sabretooth: the power sets are nearly identical, so what makes the difference?
There are a number of ways to create evil twins. Super-science is always good: the copy can be an actual clone (Galatea from JLU) or a mysterious copy (Bizarro or the Sand Superman from the 1970s, for instance). Alternate timelines, mysterious future timelines to be avoided, those are good sources. Dark mystic mirrors will do. Transporter accidents. :( (Keeping the transporter is the precedent for keeping the holodeck.)
Defeating evil twins usually requires pointing out the flaw in their thinking, the One Thing that the evil twins lack that the heroes have. It might be external ("We have the Flash!") or it might be internal ("We still don't kill, and this gives us the edge when we release sixteen enemies to fight you instead").