the idea can already be there, already implemented and even already successful
and an incredible execution can just take over
you don't take over on ideas, you do take over on implementation
few examples: hotmail and then boom gmail took over, IE6 and boom Firefox took over, etc.
in fact implementation is so much more important that you have to keep doing it right otherwise someone else take over
also you can not keep an idea secret forever, once you release your product, everyone know about it and get a shot at their own implementation of your idea