If baptism is really a symbol of one's dedication to God, as the WTS agrees it is, then why would someone who was baptized years earlier need to be re-baptized after studying with Jehovah's Witnesses? In discussing the two questions asked of baptismal candidates, the Watchtower, April 1, 2006, page 22 par 4 says:
Therefore, those who qualify for baptism have usually studied the Bible for months or even a year or two, so that their decision is neither hasty nor ill-informed. At the baptism itself, the candidates answer yes to two key questions. Since Jesus emphasized that ‘our Yes should mean Yes, our No, No,’ it will be helpful for all of us to review carefully the significance of these two baptismal questions.
Re-baptizing someone who has already been baptized undermines this point.
In ancient times it was customary to rename a person to show your dominance over him. I guess re-baptizing a person the the name of Jehovah's Witnesses serves the same purpose.