I was experiencing slow uploads to the CDN, so I was digging around in the code, and found that when you upload a new image to the Media Library, every image (including all size variants) are uploaded twice.
This is because both
add_attachment hooks are fired when you upload a new image. See http://core.trac.wordpress.org/browser/trunk/wp-includes/post.php#L3649 (which is in
update_attached_file() is called then the
add_attachment action is called 9 lines later.
This also happens for all attachment size variants. I presume w3tc could only hook in on