On his podcast, David Perrell made the point that just because an idea is out there, that doesn’t mean that everyone has read about it or knows about it.
This reminded me of a line that I heard on Back to Work from Merlin Mann: Every day a child is born who has never seen Star Trek.
Things are not known once and for all forever. Classic stories are always going to be there to be discovered by a new generation. Nothing is every going to be so well known that it never needs to be re-presented.
What does this mean for you, the writer? Don’t stop yourself from writing about something just because it has already been written about. Write about it and it will be fresh for someone.