Having to Login twice in the Joomla Admin pages.

On a few of our site-Developments, we experienced the issue, where you need to log-in Twice to get in the Joomla back-end.
No big deal right? But it still is a sign that something is wrong and needs to be fixed.
What we found is: A conflict between the .htaccess settings and the variable settings in the configuration.php caused this behaviour.

Often when going through the configuration settings in Joomla, the site variable is set to the domain name only, without the "www" prefix. Although everything seems to work on the front end without issues, it will cause issues in the back-end.

If you do the proper Rewrite settings in your .htaccess file, make sure you do add the "www" in your configuration.php file.

How it should be:

The .htaccess file: (fraction of it)

RewriteCond %{HTTP_HOST} !^www\.domainname\.com$ [NC]
RewriteRule ^(.*)$ http://www.domainname.com/$1 [R=301,L]

The configuration.php file (or your substitude config file, referenced by configuration.php)

var $live_site = 'http://www.domainname.com';    

NOTE:

Not adding the proper settings as outlined above, might even cause a side effect if you click the preview link in the back-end and give you the " Invalid Token problem when trying to log in".
(have seen this in a few cases but not all, so it might be in relation to a particular template choice)