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.