I caught a Pickachu, and retrieved it form the box at the Pokémon Centre. I returned to Santalune Forest and the next several Pokémon were more often Pikachu. 10 Pikachu to 10 non Pikachu. I thought they were supposed to be rare; they certainly were before I caught one.
Not normally, however, when you have a Pokemon with the ability Static in the lead, your chances of encountering electric type Pokemon increases to 150%.
If Pikachu's encounter rate is for example is 10%, it becomes 15%.
If your Pikachu isn't in the lead when you saw the other Pikachu, then you were just lucky :)