Posts appear to show up twice because the forum uses a hybrid model to display threads. I agree with Elf that, for me, this has made reading through threads (especially longer ones) far easier than it used to be.
Think of it this way. The thread will always display all the posts in exactly the order they were posted. This is how the old forum operated by default. However, this current set up includes a few extra bells and whistles to make it easier to track individual strands of conversation in a larger thread.
So, for example, you might see that a post has “X replies” to it. You can click on that and a box will pop up showing you those replies. Now, this doesn’t “move” those posts. They still exist further down the thread in chronological order. It’s just giving you a snapshot of what is to come. That way you don’t have to sift through many, many posts trying to make sense of which comments were in response to which other posts.
I like this feature a lot as I often find myself reading through longer threads and coming upon posts I would like to respond to. If I can see immediately that the person has already received a bunch of replies (and if I can further read those replies and see that they basically said everything I had planned on saying), then I save myself a lot of time that I would otherwise spend repeating what has already been said. Conversely, if it shows that no one has replied, then I know right away that the question has not yet been addressed.