When you check the long grass for a pokemon, you get one, two or three exclamation marks.
!!! exclamation marks means you have the flying OR surfing pikachu.
(!! has a small chance of being surfing/flying, but a larger chance of having volt tackle or no special moves. It's impossible to actually tell).
Only one - flying or surfing - will appear each time you stroll. If you don't reset the course, every !!! will be the same as the first one you find.
After you catch your first !!! pikachu, choose 'receive gift' instead of 'return from stroll'. The pikachu will end up in your PC when you check it, but you'll still be on the stroll. You can decide from there whether to stay on the same course and catch more of the same, or reset the course to find the other one.