Unfortunately, many of you didn't understand my reason for opening the thread, it's beyond the writer. I PM people when I see their errors to fix it, so it's nothing. But this thread, even if it's my post that had that error, I would have done the same. Did I blame the writer for it? I reported an error, not the writer, I too can make that mistake, so stop painting it as if someone targeted anyone.
You don't need to PM them but you are free to do it if you want. If after sending them PM and they did neither reply you nor delete their repeated post, you can report those posts to moderators. I am very sure that moderators will delete such repeated posts.
Above replies show you a correct point that it is not a forum bug, but it bad posting style from user side. Forum has a warning message that is in red color so it is noticeable enough. Posters ignored those warnings and continued to post, but it's not too bad. After they clicking on post, and their post was actually published, I believe they ignored one more step: don't read their post after posting.
If they spend some seconds to glance (no need to read) at their published post, I believe that they will see duplications and delete one of repeated posts.
Both steps are all from user side and posters can change their posting style to make their posting better.