Sounds like you got yourself a hermie!



Can you post any pics? Someone should be able to tell you for sure.
Your options are to either chop it off, and HOPEFULLY avoid it pollinating your other plant(if you have seeds on one, then chances are that the pollen has already made it to the other)... Separate it from the other plant and let them flower on... Or, let them both go full steam as is. You're going to end up with seeds in both plants.