Inform both parties, and then tell each one the following:
I will owe the one who can show true ownership on the contract. Until then, such confusion is deemed a breach by each party and I do not know anyone until the matter is cleared because I am not legally required to owe double.
They freak out and someone will pop up with the paperwork.
Otherwise, write to the new owner, make payments to them and cc the letter to the old owner.
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I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.