How to divorce a bigamist

Two relevant cases on bigamy in New York: Aldrich v. Aldrich, 4 Misc. 2d 353; Donohue v. Donohue, 57 AD 2d 543
You “divorce” a bigamist by getting the marriage annulled. A bigamous marriage if void ab initio which means the marriage was a no go from the inception. It is illegal, in other words, to marry someone if your marriage to someone else has not yet been adjudicated caput by a Supreme Court judge.
You don’t need to prove anything to get a bigamous marriage annulled, only that there existed another valid civil marriage between your spouse and another person at the time your spouse married you. However that may not be as easy as you think. Either party to the second marriage or from the alleged first marriage that was not properly terminated can bring an action for annulment and can “collaterally attack” the second marriage.
For example, let’s say Jerry married Sue in 1970. That marriage was never properly divorced or annulled (even though Jerry may have thought so). If Jerry goes out and marries Mary in 1999 and Mary finds out  in, say, 2001, that Jerry and Sue never got a divorce, Mary can move to annul the marriage Jerry. Jerry can also move to annul the marriage to Mary. Sue can move to annul Jerry and Mary’s marriage.
This can get messy if the two marriages are fighting over property. I mean, which marriage owns or is entitled to the house? Because if, in the above example, Jerry buys a million dollar house in 2000 when Mary was supposed to be his new wife, he may have an issue with Sue who was really is wife — his only legal wife. In New York anything earned during the marriage is “marital property.” So since Jerry and Sue were never divorced, if he bought that house in 2000, that house belongs to Jerry and Sue.
But Mary may have some interest in the property too, but it would get complicated…
BTW, bigamy is a class E felony in New York. Penal Law 255.15.
Bigamy can occur innocently, by the way. A party for example, may think they got a quickie Mexican or Dominican divorce, but it may not have been a final divorce decree. In which case any remarriage would be bigamous and void.
Even if your marriage occurred right here in New York and the divorce judgment was signed but you remarried before there was an ENTRY OF DIVORCE in the county clerk’s office? You have a bigamous situation on your hands. And your second marriage is subject to “collateral attack” and you could do some prison time since it is a felony. But don’t worry. So long as you can show that you honestly believe you were divorced, usually the prison thing doesn’t kick in. In other words, a “reasonable belief that you were unmarried” is an affirmative defense to bigamy. Penal Law 255.20.