How can one be born again and then be unborn again?
Sure brother, I think we can. We call it death.
Being "born again" is a physical analogy that describes a spiritual rebirth to a new life. Jesus defined "eternal life" as a relationship, as knowing God.
We enter this relationship willfully, of our own choosing, and in accord with His criteria (e.g. belief, repentance, confession, etc.).
We can exit the relationship in the same way - willfully.
This is why we are admonished throughout the New Testament to endure, persevere, to "work out our salvation with fear and trembling" until "we see the day [of His return]" because of the DAILY temptations, persecutions, and accusations thrown at us by the devil and his minions to get us to stumble, to question, to leave the Lord.
That we are thus admonished naturally implies, nay
requires the possibility we may NOT endure, NOT persevere, NOT work out our salvations with fear or trembling.
That we still can sin, and indeed do sin requires the possibility that we can become hardened by the deceitfullness of this sin - otherwise there would be no need to admonish us in our behaviors.
If we cannot fall away,
- Why do we still sin? And why all the admonishments against sinning as Christians?
- Indeed, if we cannot fall away, what then is the purpose of sin in our lives (given we do still sin)?
- God is not tempting us to sin, satan is. Is God then using satan to somehow discipline us?
- Is it God's intent we "learn" something in our sin?
If we
persist in our particular sins, we can become hardened by them, hardened to the point we can eventually retract our decision to serve Christ as Lord. This is why we must take sin seriously, why we must fellowship regularly with other Christians, regularly read our Bibles, regularly pray to God, and regularly strive to do His will in our lives and serve as Jesus served.
No, "once saved, always saved" is a delusion and not of the Lord (who cannot lie). Does He strengthen us, empower us, provide us ways out when tempted? Absolutely! Amen!

But we should not be naive to the possibility we can become hardened by sin too, should we persist in it.
To be absolutely clear, the Christian, indwelt by His Holy Spirit, is no longer ruled by sin - sin no longer has absolute power in the Christian's life. This is why we are to "consider ourselves dead to sin" and "alive to Christ." With the Holy Spirit in our lives, we have the power to overcome sin in our lives - and with every temptation, God provides us a way out.
But this is a hostile world - as we all know too well. And until we are safely in His arms in eternity, we must be ever vigilant:
"Humble yourselves, therefore, under the mighty hand of God, that He may exalt you at the proper time, casting all your anxiety upon Him, because He cares for you. Be of sober spirit, be on the alert. Your adversary, the devil, prowls about like a roaring lion, seeking someone to devour. But resist him, firm in your faith, knowing that the same experiences of suffering are being accomplished by your brethren who are in the world. And after you have suffered for a little while, the God of all grace, who called you to His eternal glory in Christ, will Himself perfect, confirm, strengthen and establish you." I Peter 5:6-10
May God's grace be with us all as we strive to serve Him in all faith and perseverance.