The truth is, anything you can think of has been done, even if we aren't aware of it. Whatever original idea I may have come up with, I can bet that someone, somewhere, in some far corner of the planet, has already thought of it. It doesn't necessarily mean I'm unoriginal, it just means that most everything has already been executed in at least one form of media.
Having come to terms with this fact, I don't let the 'it's been done' roadblock get in my way anymore. So what if an idea has already been done? You can always put a fresh spin on it. Even if that fresh spin has already been done, it will still be different in some way if you let it be. Don't let the fact that your idea isn't as original as you thought it was compromise your creativity.
If all else fails, you can always use an idea that has been used so many times, people would never expect anyone to touch it with a ten-foot pole, and claim that you are using it ironically.
