This question is brought up in a new documentary film, "Donor Unknown," In this documentary a 20-year old woman who has never met her donor/father (Donor 150) turns to a website to try to find her biological father and finds that she has more than a half dozen siblings with the same anonymous father. After reading a newspaper article about the search, Donor 150 decides that his children have the right to meet him. By the way he turns out to be a self described beach bum who does odd jobs and lives in an RV in Venice Beach.
This brings up the question of whether there should be a limitation on the number of times a man can donate or number of pregnancies he can create to avoid unintended reproduction by unknown half siblings especially in small communities. This is a very complex issue but I feel that a donor should be limited to perhaps three pregnancies. This also will discourage "professional donors" such as Donor 150 and might result in better donor selection.