I have set English as the only language for the site each url is showing up in the sitemap twice like below:
 
mysite.com/page1.html
and
mysite.com/page1.html?language_code=en
 
This is generating duplicate content issue.
 
How can I fix it?

Just posted the same problem first thing this morning if you have ftp access then try this
 
http://www.viart.com/forum_topic.php?thread_id=3500&page=0#message_form
 
Hope it helps