Clicking Preview multiple times quickly produces multiple previews.
                    Type a comment, then double/triple/whatever click the Preview button quickly. Multiple previews will be rendered.                
                        
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I haven't looked at the JavaScript behind the preview button, but if this is truly happening to people they might want to do something like:
onlick
$('.PreviewButton').attr("disabled", 'disabled');
onsuccess
$('.PreviewButton').removeAttr('disabled');
Yep, happens for me here too.
@Tim I see two or more requests being sent to http://vanillaforums.org/vanilla/post/comment/13230 with Type = Preview in Firebug. I assume the fix suggested by @Flam should be good enough, right?