Dear

On     [date]    , this office mailed to you a check
in the amount of $       per your     [date]  invoice.

After reviewing your file, I realized that this account
had been paid in full on    [date]       I am enclosing
a photostat of our cancelled check #      in the amount
of $

I would appreciate it if you would reimburse this office
for the duplicate payment.  I apologize for any
inconvenience this error has caused.

Thank you for your prompt attention to this matter.