Obviously, you should contact your attorney and discuss this issue. It's likely that this event can be used to strengthen your case. Unless it materially affects the outcome of your case, you may want to ignore it. Although illegal, if the bigamous act occurred at the end of your divorce proceedings, it may be viewed as more of a technical violation than a deliberate, continuing act of fraud (what bigamy laws are generally intended to prevent).
Note that you don't actually have to be married to commit bigamy, a popular misconception. In some States, an unmarried person commits the crime of bigamy if he or she "purports to contract marriage knowing that the other person is married" or "cohabits in this state after a bigamous marriage in another jurisdiction". This is sample wording from the Missouri Revised Statutes, but other States have similar provisions.