Here is an interesting photo essay about people, unrelated, who look like each other. How much do they look alike? Like this:
John Armstrong, a JFK researcher, wrote a very compelling book, Harvey and Lee. You can read or download a PDF file here. The book makes the case that there were two Oswalds who, from a young age, were groomed by the CIA for an undercover operation, putting agents behind the Iron Curtain. Why two Oswalds? Read the book.
This is a collection of photos of "Lee Harvey Oswald" at various stages of his life. As you can see, the Oswalds on the top row aren't a great match with the ones on the bottom row. But if you only knew Oswald, say, as a guy working in the Book Depository you wouldn't know about someone being Oswald at the same time at, say, a firing range or test-driving a car.
(A tip of the hat to George Bailey for the link to the various doppelgangers.)