Hi, and welcome to Just Answer. Thank you for entrusting me with your legal issue. I will do my best to assist you. During the course of our interaction , I may need some additional information. Also, please keep in mind that while I assist you, I don't know what you already know about the issue unless you tell me.
Assuming that you paid December's rent......you have breached the lease. He must try to re-rent it and cannot collect double rent. However, he is within his right to keep January's rent. With the holidays, it will be improbable that he will rent the place for 1/1/09.
rich
Lawyer
30+ years NYC R.E.litigation & closings; contract law professor.