It would be a VOID marriage actually. It would be petition/summons for declaratory judgment to void the marriage in Wisconsin. It is not something found with fill-in forms. I suggest working with an attorney to do this.
You will need to hire an attorney to guide you through the process but, yes, you can file for a divorce or annulment based on the grounds he was still married.
You can either hire an attorney or file for divorce or annulment yourself. If he was married at the time you guys got married, then your marriage is null and void.
The validity of a marriage can be challenged (and thus annulled) on a number of basis (e.g. a prior undissolved marriage, lack of capacity, fraud, duress, etc). To annul a marriage, a petition has to be filed with the Court.
In Washington state, you must file with the court a Petition for a Decree of Invalidity of Marriage, which is the statutory process for obtaining an "annulment".
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