One look at the picture provided convinces me that one's chances of finding one's Double, even in a see of millions of people, is probably negligibly small. Those people are not "dead ringer"s for each other.
Pictures aren't sufficient, mannerisms and facial expressions count for a lot. I know two people who have dopplegangers identified; superficially similar in appearance but get them talking and it's really striking.
Seems like something that would be solvable by technology. Facebook has a dataset of what billions of people look like, they could probably find 2 or 3 who look exactly like me
That would be amazing (but also incredibly creepy). It would also probably end up finding a lot of people who actually are related and don't know it (adopted children etc)