What are my options after I moved out before my lease was up?

To give some specifics - I accepted a job out-of-town, so I needed to move out. I was in a 7-mo. lease; it was October 1 and my last payment would be January 1. I told them I was giving 30 days notice, so I would certainly have to pay for October, and they initially said I would have to pay November, too ("and you're lucky we're not making you pay the rest of the lease!") They decided to keep my $675 security deposit, which I desperately needed, but - oh well. They also demanded an extra $150.00 carpet-cleaning fee, even though we had just cleaned the carpet (shouldn't that come out of the lost deposit!?) Before I left, in mid-October, I found someone willing to move in November 1st, and the leasing office grumbled about it, saying they weren't sure if that would work out.

Now, at the end of November, I got a certified letter from them, saying I now owe them $2300.00, payable immediately. My question is: is it legal for them to be collecting rent twice, on the same apartment at the same time? Someone is already living there now, and they're wanting rent from them and rent from me simultaneously. They want me to pay rent for November, December, and January, as well as $100.00 late fee (and the rest listed above).