When we truly turn to God in repentance through faith in Christ Jesus as Lord and Savior, we are saved; we cannot lose our salvation.¹  As God says: "Therefore, there is now no condemnation for those who are in Christ Jesus,"²  All of the guilt and penalty of sin is removed once and for all.  God chooses to remember our sin no more as to our salvation.³

When a child of God (a Christian) gives into temptation and falls into sin, what they need to do is turn away from it (repent of it) and seek God's forgiveness.  When they do, God has said that He will forgive them their sin and the shed blood of Christ Jesus on the cross of Calvary (when sin was judged by God) continues to purify them from all unrighteousness.⁴

As to your question on Christian baptism, baptism does not save us.  God has asked His children, however, to be baptized.⁵  It's a testimony of what has taken place in the life of the one being baptized; Christian baptism is not to be repeated over and over. 

If one chooses to be re-baptized however, one who was baptized before they truly turned to God in repentance through faith in Christ Jesus as Lord and Savior, that's permissible; there is nothing wrong with this.  In fact, it's a good thing to do; when it truly means something.

In Christian baptism, we are testifying to the world that we were in Christ Jesus when sin was judged on the cross of Calvary, that we died with Him, were buried with Him, and have arisen to a new life in Him.⁶  We are also testifying that we are no longer an unbeliever, but, rather, a believer in Christ Jesus as Lord and Savior.⁷

Bible References:

¹ (Ephesians 1:13-14)

² (Romans 8:1   NIV)

³ (Hebrews 8:12)

⁴ (1 John 1:5-9; Romans 8:1)

⁵ (Matthew 28:19)

⁶ (Romans 6:3-4)

⁷ (Acts 2:38)