I've been encountering a lot of duplicate posts—a minor annoyance, but an annoyance nonetheless.
It appears that lately, clicking on the "Submit" button often fails to link to the "Your post has been submitted" screen, making it seem like a comment has failed to get posted when in fact it has.
Until/unless this issue is resolved, I recommend that commenters copy the text of their posts before submitting them (just hit Ctrl-A Ctrl-C; that's ⌘-A ⌘-C if you're a hipster). That way, if the "Your post has been submitted" screen weirdly fails to pop up, you can refresh the page and verify whether your post went through. You can try it again if it didn't, but it most likely will be there.