Sigh....
We've been here before, but ok: http://www.gotquestions.org/Jesus-genealogy.html
(If lurkers REALLY want to know, they can Google away, just like I did.)
There are two possible explanations (1) somebody made a mistake (2) some truly bizarre incestuous relationships. Only an a priori assumption of infallibility precludes one from choosing the simple and obvious explanation (1).