Is it necessary to dissolve a marriage that is not really valid if you find out the other person is still married?

Your marital status has many consequences you may not have thought if, and yes, you must divorce or seek annulment. There is no 2 year window.

In cases where deception occurred, California allows for the deceived spouse to seek a divorce instead and treat the marriage as if it were a valid union. This is known as the putative spouse doctrine and provides certain rights and protections for parties who are led to believe they’ve entered into a valid marriage through their partner’s deception. In this instance, the deceived spouse can seek a divorce and would have rights to property distribution and possible spousal support .

Yes you will need a court ordered annulment. You would need to file in the state where you reside NOT where you got married.