Simon didn't pay attention the first time it happened, and he thought it was normal the second and the third time, but when he met the fourth person in a row that sounded vaguely familiar but knew was absolutely impossible, he began worrying. Then came the fifth and the sixth person, all from different backgrounds and with a slim to none probability of having had an encounter with him before. There was a seventh person, and one after that one. And many more. Simon had lost the numbers by the time anybody he would look for more than 2 seconds became familiar.
Ending 1) Simon realised with time that it was not only people, but also things that he seemed to remember from other places. And that there was an explanation for some of the cases and he could actually remember where he had seen the person: on a rainy day, crossing the street, or in a bus, just passing by. Once he stoped an old man in the street and by talking to him finally realised where he saw him before: he was the doctor that assisted his birth. But that gift had a price: he had to forget other things in order to remember all the faces he saw. When he was 60 he didn't know who he was and spent his time walking down the street pointing at people and screaming "I know you!".
Ending 2) Simon began remembering where he had met some of the people he seemed to recognise, and he could put all the pieces together: he had been in a hospital for a very long period of time and they had performed in him all sort of operations to control his thoughts. When the shrinks came for him, he knew them already. Them, and all the other interns. Yes, they all had been in the plot, he knew.
Ending 3) Simon began sleeping more, eating healthier and doing some exercise, and the feeling of knowing people he couldn't possibly know vanished. That and his strange sense of humour. And his apetite for weird movies. And his willingness to read every book, and many other hobbies. But at least he was sane. It was better that way, he thought.
Ending n) (your turn)
I'm really interested in reading your endings because Simon... is me.