A man jumps in your way as you're walking to the post office and he says, "You call yourself a detective? Answer me this, brothers and sisters have I none, but the man I saw yesterday is my fathers son who was he?" You smile sympathetically and answer the crazy man, "He is...

The riddle 'brothers and sisters have I none, but the man I saw yesterday is my father's son' is a classic puzzle. Since the speaker has no siblings, 'my father's son' must be the speaker himself. The man in the mirror/reflection is therefore the answer - he saw himself. Twin brother, imagination, and clone are incorrect literal interpretations.