Google is notoriously one of the most (if not the most) data-driven companies in the history of the world. That doesn't mean they never make bad decisions, but I'm sure they've taken user behavior into account.
The duplication you mention isn't a bug. Just because I've seen something before, doesn't mean I never want to see it again, especially if it's relevant. Searches are often re-searches for information you've seen before. The most important thing is relevance, and your social interactions are relevant.
I think what many here are missing is that the exposure of social in search results is only the first-order effect. But the social signals themselves probably affect your personal relevancy algorithm, too. I imagine that's a valuable addition, and it will only get better with more social data.