Its called superfoetation. It's extremely rare and there's only about one recognised case a year if that. What happens is the body doesn't produce the hcg hormone straight away to let the it know you are pregnant so you ovulate again.
No, you would have to ovulate after being pregnant. I believe there was one woman who they thought had done that, ovulated twice in short order with not enough time in between for the first embryo to implant and send a signal to the body not to ovulate again. But that is the only write-up I've ever seen on the topic, and you never know (when there is just one case) what was really going on.