My Daughter Was 11 Weeks Pregnant And Started Bleeding This

My daughter was 11 weeks pregnant and started bleeding this weekend. She went to her OB yesterday, where they performed an ultrasound. It revealed that there were 2 sacks. One sack measured around 11 weeks, but unfortunately there was no fetus and the sack was "deformed". However there was another sack that measured 5 week size. It is perfectly shaped. At this point they couldn't tell if there is a viable fetus in the sack or not. OB stated that it could've been twins and that one just didn't develop past week 5 and unfortunately she miscarried the other. However, she also stated that in rare instances a mother can ovulate even after pregnant and that she could possibly be pregnant? They took blood levels and will again tomorrow. How rare is that? What are the chances that if she is pregnant with a 5 week fetus that it will be ok?