This is the message I get when trying to activate Cornerstone… Any ideas on what to do next.

Fatal error: Cannot redeclare class Cornerstone_Plugin_Base in C:\Program Files (x86)\EasyPHP-DevServer-14.1VC9\data\localweb\My Projects - Semester 4\wordpress-4.8.1-en_AU\wordpress\wp-content\plugins\cornerstone\includes\utility\plugin-base.php on line 6

Hi there,

It seems that there is the duplicate of the plugin somehow causing the problem. Kindly go to wp-content/plugins/ and delete the cornerstone folder completely from there.

Then make sure that the theme is updated to the latest version. After that go to X > Overview. Wait for few seconds and go to your Wordpress Dashboard, you should see a message that Cornerstone is installed automatically.

Needless to say to have that automatic installation you need to make sure your installed instance is validated.

Hope it helps.