It happens all the time in my town. Villagers in my town often move in exactly where a previous villager used to live, or 1 square off from where a villager used to live.
LOL it happened to me 4x where they moved in to the same exact plot. i had beau move in to a spot, had him move out so i could replot him. while i was cycling to get him back...t-bone replaced beau and plotted right there. then when he moved out, agnes replaced t-bone and plopped in the same spot. the same thing happened with the next villager who replaced agnes. one way to prevent it is to put a PWP on top of that spot if you don't want anyone moving there.
Same, twice I had a new villager take an old villager's spot (once it was a good thing, the other time it was bad). But I think it's because I covered a lot of the town with PWPs so that probably narrowed it down a bit.