First of all try to collect documentary evidence against her that she is already married with that person if documentary evidence available then file a criminal case against her under section 394/395/494 of IPC.
If there is no evidence then file a criminal case against her first husband for the offence of adultery under section 497 of IPC.
Fight the cases on merit and collect all the documents of her previous cases those documents shall be very useful for fighting the cases.
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