Assalamu Alaikum,
I wanted to marry a girl, she was a islamic minded girl. Don’t have a any relation with her. But she is now married, still now I wan to marry her. So can I do dua to Allah?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is impermissible for a man to supplicate Allah, The Exalted, to have so-and-so, who is married, as his wife; this is considered transgression in supplication. Moreover, it implies asking Allah, The Exalted, to separate her from her husband. This is endeavoring in corruption and attempting to ruin a marriage, which is what sorcerers and devils do. A person who believes in Allah, The Exalted, and the Last Day would never consider doing this. Allah, The Almighty, Says about sorcerers (what means): {And [yet] they learn from them that by which they cause separation between a man and his wife} [Quran 2:102]

Jaabir ibn ‘Abdullah narrated that the Prophet said: “Satan places his throne on the water, then he sends his troops to people; the closest devil in position to him is the one who causes the most mischief amongst people. One of them (i.e. from his troops) comes to him and says: ‘I did such-and-such.’ Satan responds: ‘You have not done much.’ (This process continues) until one (devil) comes and says: ‘I continued to whisper to him until I made him separate from his wife (i.e., divorce her).’ Thereupon, Satan brings him close to him and says: ‘Yes! You are the best!’” [Muslim]

So leave this woman and seek another wife; there are plenty of women available. Verily, goodness lies in the Hands of your Lord; you should ask Him to bless you with a righteous wife with whom you can relish happiness.

Allah Knows best.