Rats, somewhere else I posted an article -search- may turn it up, or Google-
Its not a chromosomal abnormality per say, its to sperm causing fertilization at the same time.
That's why the are noticed split left to right,
BTW the left ovaduct in a female remains and the right one never grows from pre-hatching, so if the left side is female you can have eggs.
for self-fertilization to occur (now remember she/he is genetically only her own blood sibling- her mother is the same and she has two different sets of her father's genes...)
the male track would also have to be 'hooked up' usually in these types of animals (natural genetic chimeras) only one set of organs is hooked up, even if the other set is present (creating hormone).
Also like the hybrid animals (mules as an example) there's a much higher rate of fertility in females then males even though in mixed family animals (real hybrids, not mixed breeds but like tiglons, ligers, mules, etc) the female animals can carry mommys type of animal to term sometimes although it is (so far) very sterile.