In groups and denominations that practice believer's baptism, all people who declare themselves "being born again" and who have not previously been baptized as a believer are rebaptized, as baptism is not seen as a sacrament, but as a ritual expression of an interior conversion.
Converts to Protestantism or transfers to another Protestant denomination are considered to have received a valid baptism if they have been baptized with water and in the name of the Trinity.