Usually when people become JWs, they have been baptized once and thus in a sense are rebaptized. But here it is talking about someone who was baptized already as a JW and feels the first baptism was not valid and they need to be rebaptized.
*** w73 6/1 p. 341 par. 25 Keeping God's Congregation Clean in the Time of His Judgment ***
Would there be need for rebaptism on the part of those abandoning their addiction to tobacco or other harmful product? No, this does not seem necessary. Knowledge brings responsibility and educates the conscience. (1 Tim. 1:13) The congregation gave them to understand that their practice did not 'prevent them,' and they were baptized in accord with that understanding. (Acts 8:36) Of course, if an individual feels that he presented himself for baptism with a 'bad conscience' due to such practice, he may decide to be rebaptized. That would be his personal decision.
*** w64 2/15 p. 126 Did You Make an Acceptable Dedication to God? ***
However, it is a different matter when a person was committing serious wrongdoing at the time of his "dedication" and baptism and even thereafter. If one was habitually sinning, practicing a grievous wrong during this time, even though ceasing from it some time after his baptism and making advancement in the service of Jehovah, he was in an unclean state before God at the time of his baptism. Such a baptism, since it did not follow a true dedication, would be invalid. If such a person has now forsaken that practice of sin, repented and made a sincere dedication of himself to Jehovah God, he should be rebaptized.