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 photocopy 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.