system
A chat on subreddit /r/me_irl
human
Women only have XX, Men have XY, the baby takes 1 sex chromosome (X/Y) from each parent. The mother is guaranteed to give an X (she only has X) but the father has a 50/50 chance of giving an X (baby is born a girl) or Y (baby is born a boy). The only way that you could get a YY baby is by finding a Man who, by some course of events, produces Y egg cells (impossible) and combining with another Y sperm cell.
gpt
A YY embryo would not be viable. No matter how you got one, some of the genes needed for development, and life, are on the X, so a YY embryo wouldn't live.

Edited to add, YY could be possible. Not viable, but possible. Sometimes errors in division produces sperm cells with two Y's.  XYY syndrome exists and people can be born and grow up having it. Sometimes errors in division produces eggs or sperm without a sex chromosome.  This can result in someone having Turner's syndrome. They have one X, no Y. It would be incredibly rare, but if an abnormal YY sperm fertilized an abnormal, no sex chromosome egg. The result would be YY.