I noticed something just now as I was replying to a thread and waiting for it to go through. I had another window open and read in another forum while I waited, then went back to my post to see how things were progressing (boards are a little slow today). The ‘preview’ screen was still up and I clicked on the screen, and I noticed the submit post button flickered when I did this, even though I did not hit the button again. I thought ‘aha’, I bet this one will double-post, and sure enough, it did.
Could this be one reason why the double-posts happen? If clicking anywhere on the screen while the posts load causes the post to re-submit, I can see why this happens sometimes, even when people swear “I only hit submit once!”