Lately many users post two or more messages in a row to correct mistakes or add another thought to their previous message.
Please notice that you have 3 little buttons on top of your message: (from left to right) the "edit" button, "reply" button and "reply with a quote" button. By clicking the edit button you can re-open your own message and edit your post. Clicking the reply button on other peoples' message avoids clicking the "new topic" button by mistake :)