[seszett]

> Now if the sperm cell were from the same donor I don’t know what would happen

Probably nothing special except some inbreeding with the loss of 25% of genetic material of the donor individual (each gamete containing a random 50% of the donor's genetic material). Not sure how fast this level of inbreeding would be deleterious.

[vintermann]

That's some Cleopatra-level inbreeding. The child's - or should we say the victim's - DNA would have longer runs of homozygosity than a child of incest.