I went to edit a comment a moment ago, and it completely doubled up the edit box.
A glitch in the matrix?
I don't get quite the same effect - although I briefly thought I did, due to the "Add Comment" box still being present at the same time. I notice that in your picture the Add Comment box isn't obviously present - so I wonder if it's actually the same issue, but showing itself in a different form? (e.g. somehow your Add Comment button was renamed to Save Edits)
Personally I think the Add Comment box should be replaced by the edit box.
I'm using Chrome 22.214.171.124 (35722) beta on Windows 7, by the way. Also happens with IE8 (still with the buttons being labeled differently though - so not quite the effect you're seeing.)