Duplicate content is one of the most penalizing principles to be able to rank a search engine like Google. We speak of duplicate content when content on several pages belong (or not) to the same domain and are identical or substantially similar.
This problem can also come from a site with two accessible versions of a URL, with and without www:
To solve this problem, it is necessary to systematically redirect one of the two versions towards the other in order to have one unique version of the URL consultable by users as well as bots (preferably the WWW version).
For an Apache server, you will need to include the lines below in your .htaccess file to redirect www.mysite.com to mysite.com and all associated pages to the similar version without www
RewriteEngine OnRewriteCond% {HTTP_HOST} ^ www \ .your-domain \ .comRewriteRule (. *) Http://your-domain.com/$1 [R = 301, L]For an Apache server, you will need to include the lines below in your .htaccess file to redirect www.monsite.fr and all associated pages to the similar one with www
RewriteEngine OnRewriteCond% {HTTP_HOST} ^ your-domain \ .comRewriteRule (. *) Http://www.your-domain.com/$1 [R = 301, L]