Don't worry about it, ideas are a very small part of a successful business. Apple wasn't the first personal computer company, Ford didn't build the first car, McDonald's wasn't the first hamburger fast foods business and Google wasn't the first Internet search engine. Do what you're trying to do well, give your customers value for their money, and don't break the laws on intellectual property.
I've written about this several times on my blog "Up and Running" here at Entrepreneur.com"
Ideas are Easy, Doing Stuff is Hard
Also, you'll like this one, on my Planning Startups Stories blog:
Second or third mover advantage
It's answering this same question, although it was a while ago ...
Question added to topic Starting a Business • November 7, 2007
How do I continue plans for my business or product if I find that someone has already gotten ahead with the same idea?
Two years ago, I had a great idea for a product/service and recently started developing it only to find out it already exists! But I think there might still be some hope for me. Although the website for the product is detailed, it doesn't appear that the company has done much of anything with it in terms of sales. How can I continue working on my idea?