Trying simply to logic out the question with very limited biology knowledge, I would think any egg reproducing species COULD have identical twins by way of a single egg splitting after being fertilized. I think this can even happen in plants. Fraternal, twins, where two eggs are fertilized by the same father is obviously the norm for at least part of a brood (in the case of cephs, likely not all of the brood if fertilization occurs in situ and the species is common to an area). Perhaps the interesting question would be could two separatelyy fertilized eggs be in one egg sack and I am thinking not but I don't know that anyone has ever investigated the thought.
