Ah, double posting. It happens when you post again, even though you were the last person to post in a specific thread.
Now, you are allowed to double post IF a reasonable amount of time has passed. Give it a couple days. One of the reasons why is that we need to give other people a chance to post and respond themselves. It says this right in the Rules (subsection four, specifically which every member must follow. We can delete your posts if you do so.
Instead of double posting remember that we do have an "Edit" button! If you forgot something or want to post that very same day, use the "Edit" button and do not double post.
Any questions or comments about double posting can go here can go here.