As default when you create your first Joomla site, you will have two versions of your domain name.
One version will be the one with the www at the beginning.
Example = www.mydomain.com
And the other version will be the one without the www
Example = mydomain.com
This can become a problem when you start doing SEO inside your Joomla sites because Google and other search engines will see this as two separate websites.
As a results having a duplicate content problem.
And that’s why you need to always choose which version of your Joomla domain name will be your main one.