Hacker News Reader

I'm not sure, as I didn't dig too deeply. I forget the details, but I knew the post had been reposted, since many of the (poor quality) candidates said things like "I'm inquiring about the position posted at [some-place-other-than-github]"

The answer is nothing, I get plenty of applicants who have come from elsewhere.

But SO do make it easy to detect applicants from SO by linking to their profile.

In theory the community moderation element forces a user to cite their sources or face the backlash of deletion/downvoting, etc.