“The Catholic half of my family and the baptist/congregational/Methodist other side would not be happy if in the after life the found out they were rebaptised as Mormons!”
The Mormon teaching on this is that Free Will (referred to by Mormons as Agency) overides everything else. Any baptism is without any effect whatsoever unless it is accepted by the one in whose behalf the vicarious baptism is performed. As strange as the practice may seem, it is actually biblical and is mentioned in 1 Corinthians 15:29: “Else what shall they do which are baptized for the dead, if the dead rise not at all? why are they then baptized for the dead?” St. Paul was trying to make a point here that the dead shall be raised (resurrection), and he refers to the practice of baptism in behalf of the dead to support his argument that people will be resurrected. Paul, a true scholar in doctrinal issues, is possibly the least likely of all the disciples to ever use a false practice to argue in support of something so dear to him, namely the resurrection of the dead through the power of Christ.