I have a Greninja and a Blaziken on my team as well, and the comment above mine is correct, in a version before 8 or 7, the first battle against Zetta gave him a shadow Froakie as his first Pokemon. I found a download to an older version somewhere on either the forums or some other website and was able to play until that point and catch the Froakie, then save in a specific spot and update back to the current version and continue the game with two OP starters. So, if you found a download to an older version that's safe, you could still get both of them if you're not too far in your journey/willing to restart, or like others have said, through debugging (if that's the other way), but I'm 99% sure most people got one through old save files that they ran on older versions of the game.