Is there really anything more off-putting than revealing your next, fantastic story idea only to have the listener say, "Oh, yeah. That's exactly like this one episode of Desperate Housewives?" Or maybe, "Hey, I saw that in a porno once."
Okay, probably extreme examples, but still. We do want to be original, don't we? We definitely don't want to find out that our pet story seed has been done--and re-done--to the point that it's recognizable or cliche. I don't, at least, and I suspect that's a pretty universal opinion. As authors, we want to stand out. We want to create something never before seen or imagined. It's kind of what we do.
So when you figure out that just about every story line, plot structure, or imagined saga has, in one way or another, already been done, it can be a real set-back. We learn it early on--three basic themes, thirty some odd stories in existence that everything fits into. But those are vagaries, they are generalized themes and formulas and within them, we always believe that we can still create something completely and utterly "new."
Except, I'm not so sure we can. I'm not convinced their is anything new under the sun. Also, I'm not convinced that it matters at all. Newness, you see, is overrated. Originality, however, is another thing all together. I suspect, that is where some confusion takes place.
A story can be retold. In fact, it's meant to be. Oral tradition arises out of the retelling over and over of favorite tales. Myths are meant to be relived repeatedly. This familiarity, rather than weakening the tale, gives it power. It resonates with us more deeply because on some level, we "know" it.
We are, however, easily bored creatures. We lust for newness, despite our love of comfortable old shoes, worn out jeans and t-shirts with the most holes in them from loving abuse. And so the trick becomes not to tell a new story, but to tell a familiar story in a new way. In other words, it isn't what story you tell, but how you tell it that will make it stand out as spectacular.
And of course, the way to do that is to use your own, distinct voice. Infuse the tale with your personal passions, your unique, possibly twisted view of the world. Because we as people are unique and individual, the more of your self you allow onto the page, the better your chances of hitting that original strike.
Best of luck at it!