Is it permissible to marry a woman with whom one has committed adultery, and then she repented, knowing that she relapsed into sin several times thereafter?

If it becomes clear to you that the girl has repented with a sincere repentance, then there is no harm for you in marrying her, even if she had committed immorality with someone other than you. For repentance erases what came before it, and the one who repents from a sin is like one who has no sin.