Baptism is a doctrine of the church of God. It is a teaching and practice which the Lord Jesus Christ established as laid the foundation of the church. Everyone who believes in Jesus Christ is to be baptized in the name of the Father, and of the Son and of the Holy Spirit. In as much as we obey other doctrines regarding other issues of life, we are obligated to obey the doctrine of baptism as believers in Jesus.
The person who has accepted the doctrines of Jesus Christ is the one who is qualified to be baptized. Baptism is not to precede salvation. Salvation is the acceptance of Jesus Christ and his doctrines. Those who believe in the Son of God (whom the Father sent to redeem the man) are those who are saved. Anyone who is baptized without being saved is not doing it rightly.
If you have been baptized and you are saved, then you don't need another baptism.