Are Anabaptists' re-baptisms really conditional baptisms?

Are Anabaptists ' re-baptisms really conditional baptisms? By "conditional baptism" I mean one whose formula is something like: "If you are not baptized, I baptize thee… etc." In other words: Do Anabaptists really accept two valid baptisms per person, or are their re-baptisms conditional on whether the first one validly occurred or not? If so, does this tradition continue among any of the traditions descended from Anabaptist roots?