In my humble opinion, any Baptism beyong a "legitimate" Baptism ie one who has been baptized in the Trinitarian Tradition is a waste of time and amounts to insignificance.
Baptizing a Baptized person only cheapens the Sacrament. It doesnt have any grace giving properties since it is unnecessary to begin with. The rebaptized person's sins are NOT forgiven again as in the first Baptism and the salvation of that person being rebaptized could be in jeapardy.
I would still like to know why some believe that being re-baptized is dangerous?
I had to be re-baptized because the "pastor" that baptized me burned down the church for the insurance money a couple years later. (Although I highly doubt they would have kept a record of it to begin with).
How about becuase there is NO RE-BAPTSIM. There is ONE Baptism for the remission of sins. The Orthodox Church does not consider those actions performed outside of her to be valid Sacraments. I think every other question about why doing "just" a Chrismation has been answered previously.