Is re-baptizing, for the purpose of rededicating oneself, a church legitimized practice? If not, why not? How can one undo the rebaptism?
There is never a second Orthodox baptism.
Second Baptism is considered a second crucifixion and death for the Lord Jesus Christ because Baptism is death and burial with the Lord Jesus Christ (Romans 6:3,4; Colossians 2:12).
Regarding your question, can it be undone? If anyone is baptized for a second time in the same faith, it is considered null, but they need to repent through confessing the falseness of the doctrine of the second Baptism.
Here are Biblical verses that support this Orthodox Doctrine:
Ephesians 4:5 states, "One Lord, one faith, one baptism"These verses very plainly tell us that the Lord Jesus Christ dies no more, for by His Holy death, death died and He lives eternally.
The Holy Book of Romans 6:9-10 states: "Knowing that Christ, having been raised from the dead, dies no more. Death no longer has dominion over Him. For the death that He died, He died to sin once for all; but the life that He lives, He lives to God."
The Holy Book of Hebrews 6:4-6 states: "For it is impossible for those who were once enlightened (Baptized), and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, if they shall fall away, to renew them [re-baptize] again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame."
The Lord Jesus Christ was subjected neither to death nor to sin, yet He voluntarily died to sin for our sake. In the Lord Jesus Christ, through the power of God, we can choose to "die" to sin in the Holy Sacrament of Baptism.
Just as the Lord's death was once; and now He lives forever to God, never dying again any more, so is there never a second baptism. We hold fast to our one baptism and continue in communion with God capable of living righteously.