I am seeing the same issue with duplicated comments as in Bug? Duplicated comment (unresolved), in http://stackoverflow.com/questions/7185857/how-to-what-added-in-sys-path-append but I have some more information which might help to recreate the bug.
There was a comment which I was going to flag as offensive, unfortunately I accidentally clicked upvote and then I didn't seem able to flag it again after that. I went 'back' on the browser and flagged it. Then there appeared a duplicate comment, both of which I appear to have upvoted, but only appearing when I'm signed in. When I log out it's gone, and when I log in again it's back.
By the way, it was slightly annoying to have cast a vote like 'this is a great comment' when I was actually intending to say this is an offensive comment. Could it be possible to undo comment upvotes, like you can with Q and A posts, or at least not remove the vote to flag?