Something else really crazy and bizzare is chimerism. (see
Lydia Fairchild - Wikipedia, the free encyclopedia) It's when two seperate fertilized eggs join together (basically two seperate maternal twins fusing together to form one person)...
These people have been known to have two sets of D.N.A. and as someone born and raised pro-life it was hard to come to terms with this, and has raised some serious questions when thinking about humans in the early stages.
I always felt life started at conception... So when two seperately conceived fetus' join together to form one person... is that two people? Makes my head spin sometimes. There's so much ambiguity out there it drives me insane.