I've noticed double and triple threads/posts lately and I myself have accidentally double posted a reply thinking my first reply didn't actually "post" (the WYSIWYG editor typically disappears and shows my new posts but didn't do it that time so I clicked the "Post Reply" button a second time causing the double post)
I think the "Create new topic" and "Post reply" buttons aren't doing what they usually do after being clicked, which is suppressing the WYSIWYG editor and then show the thread with your new post like they usually do when they're pressed. Seems server/AJAX-related?
Or maybe it's just me?
EDIT: Actually I just got a 500 Internal Server error when I posted this thread and clicked "Create new topic", but it looks like the thread was still posted in spite of the 500 Error.
EDIT 2: Yeah it wasn't just me:
I just encountered a similar problem uploading that image where it looked like it failed uploading and spit out an error but actually did get uploaded, too.