>When you are posting a second reply to the same message, a page is displayed indicating that "I already created this message".

Basic validation of not allowing a duplicate if effective, of course.

>What if you want to put some additional comments or you missed an important note?

In that case, you can either reply to yourself as per the new message for adding comments or to the original message. IAC, if the content changes, that will not be seen as a duplicate and will be allowed.

Preview is also useful so see your message before sending it.