A newspaper carries a story today concerning a fertility clinic. One of the clinic's owners donated sperm and it is estimated 600 children have him as their biological father. All this happened many years ago. There are strict limits on the number of families for whom a donor may provide sperm. The position is that a donor may provide sperm for up to ten families. The reason for this is to diminish the risk of two children with the same biological father having offspring: with all the genetic risks that can entail.
Where I live I know of couples with children not one of which has the same biological mother and father. Indeed the children often do not know whom are their biological fathers and mothers. The risk of genetic problems is increased considerably as relationships develop between such children.