Copy of a letter sent

I need to send a copy of a letter which was lost in the mail. The recipient knows that the letter has been lost, and that a copy is being sent. How do I distinguish this copy from the original letter so that the records and the customer have evidence that the second letter is the one received? I hesitate to simply stamp a big COPY on the second letter, and it seems to me that I have seen some polite, non-obtrusive wording on a previous circumstance, but I cannot recall what the wording was, or where it occurred on the page. Thank you for your help.