Baptism is a public declaration of one's personal faith in Christ Jesus. It indicates that you have an intention of walking with Him as His disciple. It suggests that you intend to walk in "newness of life" - Romans 6:4. It certainly has no saving merit - that is, it is something that results from a person having been saved; not that it results in a person being saved. Consequently, if a person didn't understand the full significance of what he did, if he was a believer then he gave witness to his faith in the Gospel, albeit it may not have been as meaningful to Him as it could have been. Nevertheless, the witness of his faith was given. The only occasions in Scripture of a person being baptized twice, is found in Acts 19:1-5 when some people who had been baptized by John the Baptist in anticipation of the arrival of the Messiah, finally discovered who the Messiah was - Jesus Christ. When they found out about the Gospel they were baptized in Jesus name. The point was that they were not yet in the Body of Christ since they had not placed their faith in Christ. This made baptism relevant to them.