Strange! I experienced the same problem for the first time today with MX8 and don't know what caused it. I had several repeat images when I moved a graphic. Problem cleared when I let the page and returned to it. I have noticed in the past that if you move an image too far to the left or right then you can get a duplicate image created. I've been using Web Designer since it was launched and think it is a brilliant piece of software. Hope your problem disappears when you restart.