No one is ever required to be baptized again. That’s a point Calvin makes very strongly throughout his writings. Baptism, by its very nature, can only happen once.
Now, of course, the people who want to baptize you after you come to faith don’t believe you’ve really been baptized before. They would say that they are not promoting being baptized again; they are promoting being baptized, properly, once.
There are some groups who actually do talk about being baptized again, but that is to misunderstand the fundamental nature of baptism. Baptism is a sacrament that speaks of the reality that the Christian life has a beginning. You pass from death to life. You pass from unbelief to faith. The sacrament of baptism marks that reality in a profound way for the Christian. To say you could be baptized over and over again would implicitly say that you could pass from death to life over and over again, which is not at all what we believe. So, the singularity of baptism as the sacrament of the beginning of the Christian life needs to be carefully preserved.
We believe, as Reformed Christians, that baptism does not have to occur after faith. In some circumstances, it’s perfectly legitimate for baptism to occur before faith, and yet it remains a valid sacrament. That is what Reformed Christians believe the Bible teaches about baptism. The sign and the thing signified are not identical. The sign can precede the thing signified, or the sign can come after the thing signified. The time relationship between the two is not what’s crucial.