Think about it... just because there are existing competitors doesn't mean there's nothing for you to learn. In fact, learning about those competitors is part of the Customer Development process. How are you going to differentiate your offering? Here's a hint... having a better handle on the problems your customers need solved, gives you a leg up on providing a better solution.
Anyway, if you go through the CD process, as laid out in The Four Steps to the Epiphany, you'll find that a big part of it is about learning the state of the existing market, making decisions about market type, positioning, etc.
The other point to realize, as @sgblank himself points out, is that if you already have a lot of knowledge of the area you're working in, and if your existing ideas are mostly right, the CD process will just go very fast... you'll basically just being quickly confirming what you already know. Just beware of confirmation bias and make sure you're not tricking yourself into seeing what you want to see.