Q. 1. What does "Once baptised - always baptised" mean?
A. 1. It means that once you have been baptised, you cannot be baptised again. You can only be baptised once and it is valid forever.
The Catechism of the Catholic Church states that "Every person not yet baptized and only such a person is able to be baptized," (C.C.C. # 1246). Baptism leaves an indelible (permanent) mark on the soul and there is no way nor any reason that one could be rebaptized.
In rare cases, when the Trinitarian formula was not used during the Sacrament of Baptism, the person can be baptised a second time to ensure it is properly done. If there is uncertainty, then the priest administers a "conditional baptism." If the person was not properly baptised, than the conditional baptism becomes the formal Baptism. If the person was properly baptised, than the conditional baptism does nothing.