added code to prevent duplicate Open Graph tags when the Facebook servic...

...e plugin and Metatag Opengraph are both enabled.

Summary of the issue:
Metatag Opengraph sets a number of OpenGraph tags on each page. The Facebook service plugin for Social has to set these as well to customize the share content that shows up on Facebook when a share is made. All of this activity eventually boils down to calls to drupal_add_html_head() which, due to how the data is structured, does not check for duplicate tags. Net result, you can end up with two og:title, og:description, or og:image tags in the page header. This results in unpredictable behavior on Facebook's part as it's not clear which set it will choose to honor when scraping a page. 

This code change sidesteps the issue by checking for the presence of Metatag Opengraph and when found unsetting any of it's OpenGraph tags that would collide with tags the Facebook service plugin is rendering on the page.