Is there any reason/purpose to have a second/third/etc baptism? Does doing it once give you the privilege to just ask for forgiveness for sins committed afterwards?
This is a common question. It would be a lot easier to answer if, as some people say, baptism is not for remission of sins but simply an outward expression of an inward grace. But because the Bible says baptism is for the purpose of removing sins (Acts 22:16, Acts 2:38) the question arises whether it is once for all time or whether it needs to be redone after each sin. Logically, if baptism needed to be repeated after each sin, we should all be rebaptized daily.
Let us look at one circumstance in which one who had been baptized was caught in a sin, and see what was required of him. Look at Acts 8:4-25. There was a former magician named Simon who was baptized (verse 13). Later he saw that the apostles could grant others various miraculous gifts through the laying on of their hands. He, perhaps thinking of his former profession, sought to buy into that power. Peter rebuked him for trying to buy this power which was not his. Then Peter told him, "Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee." (verse 22) He did not tell Simon to be baptized again. He told him to pray for forgiveness.
I think this relates to a concept presented in 1 John 1:7, which says, "But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin." According to those who know Greek grammar better than I, it should be better translated "the blood of Jesus Christ his Son continually cleanses us." Once we are in Christ through baptism then all we need to do to avail ourselves of that forgiveness is to repent and ask for the forgiveness. Walking in the light doesn't mean that we don't sin, but that sin is no longer our master. We died to sin and were buried in baptism (Romans 6). As long as we are participants in that new life, even if we stumble, we have the forgiveness. Baptism doesn't make it so that we never sin, but it does make it so that when we do sin God can still cover that sin and forget it.