Mail duplicates photos

Does it have anything to do with the message having possibly
been Forwarded at some time?

Yes and when this occurs the original message was composed in HTML or received and forwarded by someone using HTML for message composition. HTML figures into this somewhere along the line.

Would choosing NOT to display remote images in HTML messages
in the Mail preferences stop this?

No and you shouldn't automatically render HTML regardless as a security feature to prevent notifying a spammer that your email address is valid. If a spammer uses HTML for message composition and includes embedded images or objects that must be rendered from a remote server to be viewed and these are rendered, you are notifying the spammer that your email address is valid or "known good" causing even more spam to be received.

If a spam message that includes remote images is correctly marked as junk when received, no images or objects will be rendered from a remote server as a security feature regardless the setting but not when a spam message that includes the same is not correctly marked as junk when received which is why you should not automatically display remote images in HTML messages.

http://docs.info.apple.com/article.html?artnum=107401

The majority of email problems would be eliminated if Plain Text was used for message composition instead of HTML.

You're welcome.

I don't know what will stop this from happening but it definitely would not happen if HTML was not involved. I have never experienced the same when receiving an original non-forwarded message.

BTW, it does not happen with the Mail.app only. I access my .Mac account as an IMAP type account with Windows Outlook Express at work and I received a forwarded message this week with the attached photos duplicated when the message was viewed with OE and the Mail.app so this is not unique to Mail.

I have seen this problem many times with Mail. And when I look at the same email in Thunderbird, or the Gmail web interface, the email looks fine. This problem has existed for years. That OE has the same problem doesn't excuse Apple for allowing this bug to remain for so long.