Well you'll all be interested to know that my mother in law was actually baptised twice.
Apparently she didn't properly "dedicate" herself to jehoover the first time. She left "the truth" and moved over sea's to marry....but was never disfellowshiped. Years and a couple of kids later she began attending again in the other country, decided she needed to get re-baptised and for some reason it was allowed.
So there we go you can get baptised twice! Why you'd want is another matter!