I didn't read the link but I have a gf who's ex bf was the father of one twin-and the other twin wasn't his. It's rare but I guess if you've been busy and you've got more than one egg it's possible. He acted as a father to both the kids, since well-as you can imagine thats a hard one to explain to the kids.
I feel like there was a poster here at one point who talked about ovulating twice in one month. Similar, but different. Bodies are amazing and they do unpredictable things sometimes.