"Duplicate content" means having the same content on several different urls. Search engines do not appreciate this because they see it as an attempt to improve ranking. They therefore penalise websites with duplicate content. In order to avoid this type of problem, we suggest that when your site works correctly in HTTPS, you should redirect HTTP content to HTTPS. This will mean that your visitors will be automatically sent to the HTTPS address and only one address will be available for the same content. Here is an example redirection to implement within an .htaccess file at the root of the website:
RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://www.votredomaine.fr/$1 [R,L]