Who cares if something exists? The important question is can you do it better?
Once you get the urge to do your own thing an important stage is to reflect upon your ideas and their feasibility. Once you hit on the right one, you should get a "this is it" feeling. Seeing other sites doing the 'same' thing should just spur you on.
Agreed - IMO, until 2 or more players have saturated the market, competition isn't going to necessarily keep you out.
Two other bits of advice:
1. Find something limited in scope, and build it under constraints. At least to start, assuming that you're going to keep your day job. In other words: can you build something that actually has value, and that you would use, in 40 hours? This is a good exercise for a first startup project, and can keep you from running down a rabbit hole.
2. Decide if this is a side project or a full-time job. If it is a side project, you can build something you like and help it grow slowly, without worrying about your take-home pay right away. If you're going to quit your day job, the stakes are much higher - you can't rely on slow growth (because your rent/mortgage depends on profit), and you should never rely on explosive viral growth (because it probably won't happen to you, no matter how good you are).
Definitely. Most innovation is incremental, not revolutionary. And, in fact, revolutionary innovation is incredibly difficult to pull off. Better make an incremental innovation first to learn the ropes and make some money to fund the revolutionary innovation that you may decide to go for later.
Once you get the urge to do your own thing an important stage is to reflect upon your ideas and their feasibility. Once you hit on the right one, you should get a "this is it" feeling. Seeing other sites doing the 'same' thing should just spur you on.