I have the same situation, I think you have a problem when:
- You clicked Back when you received the message "The last posting from your IP was less than 90 seconds ago. Please try again later."
- Wait for about 1 minute and then Post again
=> Failed!
- You will receive the message: "You already submitted this post ..."
The system thinks that you have submitted the post.
Solution: copy your post to somewhere. Get out from problem then start over with New topic/reply. All problems will be resolved.
However, I still hope admin will solve this problem so that members can create a topic/reply more easily.