ADN has a finite amount of combinations, even if there can be several hundred billions.
I don't find these "time Travelers" so rare to find, based on my first affirmation. Lots of different era lookalikes have already been found, but we only ear or read about the famous ones.
In that note, I quite often travel back 1h (France to Portugal) and then forward 1h (Portugal to France)! I'm a freaking time Traveler! Although there's nothing I can change in the hour that I go back... In time