Half-Identical Twins: Much more common in the Eldraeverse than in real life, while still not exactly common, since this sort of minor zygote-split with sex-chromosome flippery is very much on the list of Easy and Fun Genetic Manipulations You Can Do At Home, or at least if you think it sounds neat.  And yes, arguably, this makes them a type of Opposite-Sex Clone.

(NOTE: Yes, I know the trope name doesn’t match the scientific definition of the term. Don’t write letters.)