WPSSO is packed full of essential and unique features — the duplicate meta tag check being just one of many examples. When the “Check for Duplicate Meta Tags” option on the Advanced settings page is enabled, WPSSO will crawl the front-end page after saving a Post / Page and parse its meta tags. If it finds duplicates, WPSSO will issue a notice for each one. It’s important to fix these since some social crawlers — like the Facebook crawler, for example — will reject all meta tags in the webpage if it finds any error.
Duplicate meta tags can come from two possible sources:
Some SEO plugins — like Yoast SEO for example — can also add a few social meta tags, which creates a conflict with those from WPSSO. WPSSO checks the settings of a few popular SEO plugins, and can display instructions on how to disable those conflicting meta tags. Social sharing and/or Facebook related plugins can also add duplicate meta tags to your webpages. Although these meta tags are not essential to their sharing features, these plugins may have to be disabled unless they have options to disable their meta tags.
Themes can, on occasion, have options in their settings to add social meta tags. Some themes also include code in their header.php templates to include a few social meta tags. In this case, you’ll have to edit the header.php file(s) and remove the meta tags yourself. If this is the case, you should also contact the author of your theme and inform them of the problem — social and SEO meta tags are plugin features, which should not be hard-coded in theme templates. At a minimum, theme authors should offer options to enable/disable those meta tags from their theme settings page.