Anyone know any good sci-fi-ish twin/sibling/clone characters?

Me and my identical twin sister tend to go to different Cons in matching cosplays. Taking advantage of the fact that we're actual, identical twins. It's super fun and people seem to love it, so.

Just a week ago, we were at a local, semi-big Con that has a different "theme" each year, and it was revealed that next year's theme is going to be "Masters of the Universe". The trailer showed clips of Star Wars, Star Trek, various Marvel movies, and various sci-fi stuff.

We don't [i]have[/i] to stick to the theme, but we really want to. We're not sure about who to Cosplay, though... Any good ideas?

So far, we've only written down Season 1 Shiro/Season 3 Shiro from Voltron.