Yeah, I just posted that post. At first it didn't show up, but when I went into the General Automotive-section and opened the thread again the video has been posted twice and the forum auto-merged the posts.
When I posted this very post via Quick Reply I received the same message as Bobele23 as well.
One more thing: even though in my General Settings (o7) the setting for the Default Thread Subscription Mode is on "Do not subscribe" a thread is getting subscribed when I post a reply, it doesn't matter whether I use the Quick Reply or the advanced editor.
When you change the setting, save it and change it to "Do not subscribe" again it seems the bug is gone.
I was using Facebook's HHVM to get better speed, and because I was having some issues with regular php. I had it set up to fall back to regular php on failure, but it appears that somewhere late in the posting process (maybe in the redirect at the end), hhvm was failing, so the post would be tried again with php, resulting in duplicate posts. I fixed the issues with plain php and disabled hhvm, so it should work normally now.