2. Meeting Yourself at Different Points in Time
If you think of time as a ball of twine, where all possible outcomes are overlaid upon one another and whatever you are experiencing is simply a certain thread you have chosen to follow, rather than a linear progression then its not that surprising that at some points a time traveler would meet themselves, as disconcerting as that might be. Amy has met herself more than once. In Girl Who Waited(series 6) she encounters an older, embittered version of an Amy who was left behind in a hostile environment and spent decades fighting for survival. There are consequences to the paradox this creates however, as when younger Amy is saved, older Amy ceases to exist. Amy also meets herself as a child in The Big Bang (series 5). Rory watches himself die as an old man in Angels Take Manhattan (series 7), then creates a paradox by throwing himself off the roof and dying again on the same night. The idea of a paradox is played a bit fast and loose in Doctor Who, sometimes it happens and sometimes not. Perhaps it takes a critical event, like a death to trigger one. Certainly there would be interesting repercussions if multiple versions of something occupied both the same space and time but who knows what that would be. I dont blame Doctor Who for trying out different permutations of the idea. Thats part of the fun.