I think the double posts happen when you take more than about five minutes to post a reply. You enter you post, hit "post quick reply", if it's been more than five minutes, you have "timed out", and it directs you to "not post within 30 seconds" , but in the mean time, your post was actually submitted. Meanwhile you wait 30 seconds, hit submit, and it submits you post again. Next time you get timed out submitting a post, do not hit submit again and you will find that your post was actually submitted.