Concerning re-baptizing being a sin -
Well I had to ask after about an hour of searching and not finding what I had remembered reading.
I got several answers but this one was the most direct so far, from the Apostolic Constitutions. Anyone interested to read, please keep in mind that at that time there was only the Church, and various heretical groups, and that is who is being talked about.
Apostolic Constitutions, book 6, ch 15:
"Be ye likewise contented with one baptism alone, that which is into the death of the Lord; not that which is conferred by wicked heretics, but that which is conferred by unblameable priests, “in the name of the Father, and of the Son, and of the Holy Ghost:” and let not that which comes from the ungodly be received by you,
nor let that which is done by the godly be disannulled by a second. For as there is one God, one Christ, and one Comforter, and one death of the Lord in the body, so let that baptism which is unto Him be but one. But those that receive polluted baptism from the ungodly will become partners in their opinions. For they are not priests. For God says to them: “Because thou hast rejected knowledge, I will also reject thee from the office of a priest to me.” Nor indeed are those that are baptized by them initiated, but are polluted, not receiving the remission of sins, but the bond of impiety. And, besides,
they that attempt to baptize those already initiated crucify the Lord afresh, slay Him a second time, laugh at divine and ridicule holy things, affront the Spirit, dishonour the sacred blood of Christ as common blood, are impious against Him that sent, Him that suffered, and Him that witnessed."
ANF07. Fathers of the Third and Fourth Centuries: Lactantius, Venantius, Asterius, Victorinus, Dionysius, Apostolic Teaching and Constitutions, Homily - Christian Classics Ethereal Library
It should be obvious that if a person committed the bolded part, they would commit serious sin. But just as I said, it is the priest who bears that sin, not the one being baptized.
However, the part in blue explains why someone who has received a valid baptism should not go on to receive another baptism by those who have wrong beliefs (again, at this time, it was leaving the Church and joining oneself to heretics). It is an annulment of their first baptism. It does bear careful thought.
These things I know were discussed st different times and places and there are various documents, but I'm sharing this one a priest sent me in answer to my request for a link to demonstrate that rebaptism is considered a sin by the Church.