You are using addCustomTag() to set social share meta tags and that makes it challenging to unset() duplicates such as sitename. Joomla supports the property attribute in setMetatData() and the tags within that array are easily unset by a system plugin.
In future updates, please change all the social meta settings from
PHP Code:
$document->addCustomTag( '<meta property="og:site_name" content="'.JFactory::getApplication()->get('sitename').'"/>' );
to
PHP Code:
$document->setMetaData('og:site_name', JFactory::getApplication()->get('sitename'), 'property');