Forbidden by whom? Orthodox, Catholic or what? I think one baptism should suffice, but if someone wants to do it again, what is the theological basis to forbid it? It seems to me it would be like renewing your marriage vows with a different person officiating.
Forbidden by any church that considers the First Council at Nicaea binding. So Anglican , Lutheran , Presbyterian, Eastern Orthodox, Roman Catholic, Assyrian Church of the East, Armenian Apostolic, Oriental Orthodox, and a few I'm sure I missed.
"One baptism for the remission of sins" which must be Trinitarian to be valid.
It seems to me it would be like renewing your marriage vows with a different person officiating.
That's also forbidden in most of the above churches. Although who knows what the Anglicans, Presbyterians, or the more liberal Lutherans actually practice these days.
You don't "re-do" the sacrament of Holy Matrimony.