lines as long as you stay in the posting without returning.
But when I click "update message" without changes it just says "this message was already posted".

Perhaps your computer works differently and it posts the identical message when you click "ok" without changing a single letter.

But then you can prove it by yourself.
Don't close the last window, open a second one and go to the front page again.
If it shows a double posting you can go to the still open twin post and delete all content, and click "update message".
This should eliminate the twin.