We don't have a way to detect if tags are being set some other way and prevent our tags from being set. The best suggestion I have for now is to replace the $doc->setMetaData().. tags you're using now with {SCOpenGraph type=xyz} tags. Any of those tags in the output will automatically be converted to the right OG tag.
For the next release, v4.3, we'll be adding a new Open Graph plugin system. With that, you'll be able to create plugins for individual components to automatically insert OG tags on a per-page/per-component basis. The new system will also allow for easy Open Graph actions ('John read xyz', 'John listened to some song', etc). That release should be out in October, and may help you with your customizations as well.
Hope that helps explain, but if you have any questions/feedback, let us know!
Alex
Overriding Open Graph tags
