Since Doctor Who is a show that has been on and off TV since 1963, the main character has been played by 14 different actors. If the character is mortally wounded or is dying for some other reason, his body is able to "regenerate" into a different actor.
Since he is a time traveler, he sometimes runs into different versions of himself.
This show was epic about 10 years ago. Then it went into a slow decline. However, the newest season seems to have turned it around.