Any idea you have is had by someone else. True. But, your idea is a little different. Your idea is a mutation. We need the mutations.
The idea that any good idea is already known to someone else (and they probably have a startup around it already) can be discouraging. But we should be thinking about it differently. We should be thinking about it like evolution, and it's OK to have your own mutation of an idea (even if it's not mutated at all - evolution has lots of examples of successfully copying the genome of something, and you don't always want the mutation rate to be too high).
This is the essential problem with patents in the software industry - they discourage mutations (that is to say, patent holders want to make it seem like all small mutations on their idea are covered by the patent).