How can a woman, already married, legally marry again?

Sir, I am unmarried. There is a woman who got married 4 years ago by force, and now she wants to marry me. She has 2 children. How can this happen? I refuse, but she says she will do something. She has frequent fights with her husband. How can the marriage of a woman who is already married be legalized in court? Someone in my family has also gone through this before.