You don't double post by clicking the Edit button. Even when the server messes up, that's impossible.
What I noticed in most of your double posts is that they contain a quote, so you probably click the Quote button (

instead of the Edit one (

).
Double posting means that you post more than one reply in a row, which is not allowed in most of our forums.
To prevent that, either wait for someone to post, or click the Edit button (

), then add the message your forgot to the original message. (You can even delete your messages there if you accidently posted twice.)
If you can reply and make threads, editing is just as easy, so you shouldn't have any problems doing it.