Google should not create duplicate content as google was the one that proposed the #! convention for ajax content. Is it possible that you have your cache on?
Google knows that there is ajax content on the page and it should index by replacing #! with _escaped_fragment_=ccomment When it index the comments than it changes the _escaped_fragment part of the url with #!ccomment . This way when people search, google can lead them to an article containing the text on page 30 of the comments for example.
the only reason this not to work is the joomla cache which would serve the same content even though google is trying to index the ajax version of the page.
Regards,
Daniel