how came a person can only be baptised once
The Bible teaches us that Baptism is a new birth where our old, corrupted nature dies and we are born again in Christ with a new nature. Just as you can only be born once from your mother, you can only be born once in Christ. Just as you go from being a foetus in the womb to being a child surviving independently, so you can only go once from having a corrupt nature that can never enter Heaven to an incorrupt nature that can enter Heaven.
The washing away of sins that occurs in baptism is not repeated through another baptism, but through the sacrament of Confession. Thus we consider confession to be a sort of "mini-baptism", and in God's eyes, you emerge from the Absolution of Confession in just the same pure state that you emerged from the water of baptism ... like an angel!
This Q&A was found on Archangel Michael & St Bishoy Coptic Orthodox Church : http://www.stbishoy.org.au/modules/smartfaq/faq.php?faqid=88