First, UNUM probably owes you interest on the benefits they shorted you for years.
Second, it would be well-worth your time to have this matter reviewed by an experienced disability insurance attorney. For a couple hundred dollars, that attorney could probably run the numbers for you.
It's likely that UNUM is right on the overpayment--believe me when I tell you they wouldn't have let that letter out the door without three supervisors looking at it! It is doubtful that they will come back in the future and try to get this money back
But check out the interest...they've had YOUR money for years!