What about an app to find your dopplerganger by going through some normalized selfie process and auto matching? It would be a nightmare for privacy I guess.
A simple google search turns up tons of those - as you mention though, which ones do you trust?
There's a facial recognition reverse search engine that's been around for a while called Pimeyes. I think the idea of it is to see where there's pictures of you on the internet, but if you throw images of your face in there, the false matches will give you basically what you described.
Isn't this like the plot of an episode of the show Elementary?