Why store two copies? I looked in the message source and found "multipart/appledouble", which led me to finally understanding the meaning of that "Send Windows Friendly Attachments" checkbox.
Check the box and it will strip the resource fork and just send the file itself; which these days, is all that is required for most files. Uncheck the box and it sends a multipart/appledouble message which contains both the data and resource forks.
Judging by the size, Mail.app has stored a combined file based on the message, plus a version of the file without the resource fork. It also stores a file with just the resource fork. Surely it doesn't need to use 16Mb for a single 8Mb file, but I guess disk is cheap these days.
So now I understand the "Windows Friendly" bit, and note that it's possible to turn it on by default, which I have done.
