Water baptism is only an outward symbol of what has occurred in a person's heart. Water baptism does not give salvation. A person receives immersion into water because he has accepted Jesus as his personal Savior and has given his whole life and will to do God's will.
Sometimes a person will say, "Well, I have committed serious sins after I was baptized, so I should be baptized again." This is not taught in the Bible. The remedy for sin after baptism is in 1 John 1:9, "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness." If we follow this scripture, we will be forgiven. However, the scripture also states that God will cleanse us from all unrighteousness. This cleansing involves teaching and disciplining. Disciplines can be painful. Hebrews 12:11, "For the moment all discipline seems painful rather than unpleasant, but later it yields the peaceful fruit of righteousness to those who have been trained by it."
Some desire to be rebaptized because they say they didn't know what they were doing or they believe now they are serious about conversion. That may be the case. But no one completely understands what being a Christian means when he is immersed. While there is no definite scripture which prohibits rebaptism, we also find no examples of re-baptism in the New Testament.