sold my car to someone on facebook and they paid twice?

so I sold my car on Facebook to some random guy the car wasn’t paid off I owed 16,000. I only asked for 17,000. He then wrote me a check for 17,000 went to my bank and I deposited it and then they sent a check for 17,000 to the place where the loan is from and now I have 17,000 in my account and they have a check for 17,000 in their name. What do I do?