Sometimes I see a comment twice after I scroll down to load more comments and when I scroll back up, the other clone is still there. I don't think it is because of the users accidentally posting twice, because the replies to those comments show up twice too.
Thanks for the bug report. We've known about this for a while, but unfortunately it's going to take a thoughtful redesign of how we send comments from the server to your browser to make it go away. If it bothers you, refreshing the page will usually get rid of the duplicates.
I noticed that bug too