Not entirely unheard of, it's called chimerism but it is rare.
BowlofBabelfish usually deals with these assertions but she's busy with her small baby bowl.
see her response to question from recent thread:
"If you have to refer back to Y chromosomes in such cases, what about in people who also have chromosomal mosaicism, which is not vanishingly rare?"
Bowl replied, what about them? What exactly is the evidence that says that somebody with a chimerism is anything to do with trans?
It’s irrelevant. It says nothing whatsoever about transgender. nothing. intersex conditions are physical. Medically relevant errors of sexual development. They are physical, defineable, observable.
Gender identity is not. It’s an idea, a concept.
Intersex conditions are nothing to do with being trans. Nobody is a third sex. Nobody can change sex. "
thread (with many other useful clarifications):
www.mumsnet.com/Talk/womens_rights/3436868-In-defence-of-deadnaming?pg=7