Does the person you're sending it to have any way of checking if two copies of the e-mail exist on the mail server? Can sometimes do this if you have a POP3 mailbox and your ISP provides access via webmail too.
Could be that the person you're mailing is using some weird e-mail client that isn't deleting downloaded mail from the server correctly.
Could be that the person you're mailing is using some weird e-mail client that isn't deleting downloaded mail from the server correctly.