This happened to a family friend. The kicker was that the procedure was done by his own father. Coincidentally, that was also twins.
Anyway the friend immediately accused his wife of cheating. It was a big deal. Testing proved they were his kids.
“It can grow back” is the explanation. Theoretically you should get tested periodically.
Anyway, he made a joke to his father about suing him and the father didn’t take it well. I don’t think they had a relationship after that.
It is very unusual for a parent to perform surgery on a child. In fact it’s against AMA’s medical ethics.