They paid the invoice twice. They did not catch mistake, sent 1099 for that amount. Now 6 years later they want the money back. I don't know, but where I come from, if I made the mistake and discovered it 6 years later. I eat it, so to speak.
The theory of recovery would probably rest with "money had and received" which would expire on or before the 6 yr anniversary. The only claim that might survive 6 years would be fraud if it were proven the invoice for the services was fraudlently submitted a second time after the services had been paid.
You may contact Luke Wallace at Humphreys Wallace Humphreys P.C. at 918-747-5300 or Luke@HWH-LAW.com. For more information see www.HWH-LAW.com. The response above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyze this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in Oklahoma. See also avvo.com terms and conditions incorporated as if they were reprinted here.