They committed zina then they got married, and they are asking whether the marriage is valid

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Check below answers in case you are looking for other related questions:
It is not permissible to marry a zaani or zaaniyah unless they have repented
He wants to become Muslim in order to marry a Muslim woman

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Related Answers:
Her brother is going to marry a corrupt girl and his family want to deprive him of his inheritance
Family problems because a Muslim woman wants to marry a Christian man
Marrying without her father's knowledge
It is not permissible to marry a zaani or zaaniyah unless they have repented
He wants to become Muslim in order to marry a Muslim woman

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