two meta tags with all in one seo

From what I see on that theme it would not be in header.php. It would be related in the theme since it already has an SEO optimization feature built in. You need to go through the functions.php and seek SEO and remove modify this line:

$default="global|slider|layout|seo|translations";

to

$default="global|slider|layout|translations";

Which will remove the setting to be included to the options. If this does not work go into inc/settings.php and remove line 368-402 which should look like:

'seo'=>array(
        'name'=>'SEO',
        'content'=>array(
            'description'=>array(
                'type'=>'text','name'=>'description','value'=>'','ttl'=>'Site description'              
            ),
            'keywords'=>array(
                'type'=>'text','name'=>'keywords','value'=>'','ttl'=>'Site keywords'                
            ),
            'authormeta'=>array(
                'type'=>'text','name'=>'authormeta','value'=>'','ttl'=>'Site Author Meta'
            ),
            'category'=>array(
                'type'=>'check','name'=>'category','value'=>'0','ttl'=>'‹noindex› in category archives', 'hint'=>'Turn on to prevent all robots from indexing a category page on your site'
            ),
            'tag'=>array(
                'type'=>'check','name'=>'tag','value'=>'0','ttl'=>'‹noindex› in tag archives', 'hint'=>'Turn on to prevent all robots from indexing a tag page on your site'
            ),
            'author'=>array(
                'type'=>'check','name'=>'author','value'=>'0','ttl'=>'‹noindex› in author archives', 'hint'=>'Turn on to prevent all robots from indexing a author page on your site'
            ),
            'search'=>array(
                'type'=>'check','name'=>'search','value'=>'0','ttl'=>'‹noindex› in search archives', 'hint'=>'Turn on to prevent all robots from indexing a search page on your site'
            ),
            'day'=>array(
                'type'=>'check','name'=>'day','value'=>'0','ttl'=>'‹noindex› in day archives', 'hint'=>'Turn on to prevent all robots from indexing a day archives page on your site'
            ),
            'month'=>array(
                'type'=>'check','name'=>'month','value'=>'0','ttl'=>'‹noindex› in month archives', 'hint'=>'Turn on to prevent all robots from indexing a month archives page on your site'
            ),
            'year'=>array(
                'type'=>'check','name'=>'year','value'=>'0','ttl'=>'‹noindex› in year archives', 'hint'=>'Turn on to prevent all robots from indexing a year archives page on your site'
            )
        )
    ),

These steps will remove the default theme SEO which will in return get rid of your duplication meta problem. Hope this helps.