Look at the companies below and think: what they have in common?
You see, Google was not the first search engine in the market. If you belong to the internet dial-up connection probably will remember search sites like Altavista.
Apple did not create the personal computer neither the smartphone. Facebook was not the first social network site.
Making a parallel with the world of gaming, Angry bird did not invent the game of throwing things in the air to destroy others. Pop Song did not invent the game of guessing which song is playing. Draw something not invented the game of guessing drawings of others.
So, the point that I want to make is: does not matter if the idea that you want to develop already exists, what matter is your ability to develop it better.
UPDATE: I just remembered a interview of Malcom Gladwell in which he says that being first is not better. I can see the interview below (at 16m18s):