If you're a writer, you've probably been in this situation at least once: you come up with an original idea for a story, you get really enthused about it and start building on it, and then you discover that the idea isn't so original after all. It can be pretty disappointing for some of us to reach that realization, and looking at how much flack movies like Avatar and books like The Hunger Games have gotten for supposedly "borrowing" ideas from other works, it can also be pretty discouraging. Granted, both of those examples have been insanely huge successes, but not many aspiring authors have that kind of confidence in ideas they've barely begun to develop.
So what do you do in this situation? Should you abandon your idea altogether, should you alter it beyond recognition, or should you leave it as is and see how things turn out? And actually, is it even so bad to have a story that isn't 100% original?
One defense that people often make for alleged ripoffs is that there are only so many story ideas out there, so of course every book, movie, or TV show is going to remind us of something else. While I personally don't agree with the idea of there only being "so many story ideas" in existence, I will consent that we haven't come up with many new ones in a while. That being said, I do agree that stories shouldn't be condemned right off the bat for having familiar elements. What it comes down to, at least for me, is just how many familiar elements a story has and how well it presents them.
On the surface, The Hunger Games does have a similar premise to the 1999 novel Battle Royale; it centers around an annual tournament in which a group of children are thrown together in an isolated place and forced to fight to the death while being heavily monitored by the government, and (spoilers) two competitors end up defying the rule that there can only be one victor and walk away from the tournament together. Where The Hunger Games differs is that its tournament is constantly juxtaposed with a social commentary on the shallow glamor of celebrity life and how morally blind the public can be to things that are trendy. Also, the rules, setting, and reasons for the tournaments in each story are not the same, just the intended outcome.
Lastly, whereas Battle Royale is a single book, The Hunger Games is a trilogy of books that pretty much stops focusing on the tournament and becomes an entirely different entity by Part 3. What happens to the boy and girl who survive the tournament in Battle Royale? We don't know, because the story ends there. The Hunger Games sequels, however, go into great detail about Katniss and Peeta's lives after they defy the rules and how they eventually overthrow the dictator responsible for the tournament. Suzanne Collins had an idea that wasn't entirely new -- unbeknownst to her at the time, by the way -- but she gave it a unique spin and developed it into something that was able to find life and a storyline beyond its initial premise. The first Hunger Games book only sounds like a ripoff if you describe it as generally as I did at the beginning of the previous paragraph.
And then there's the film Eragon. I can't speak for the book series by Christopher Paolini since I haven't read it, and I'll give the screenwriter the benefit of a doubt too, but I find it very hard to sit through the movie without recognizing story elements from the original Star Wars film. It's one thing to have the same premise as another work; it's another thing to have the same plot. I know the idea of a lowly peasant getting thrown together with a princess, a wise old man, and a rogue scoundrel to battle an evil empire wasn't new even when Star Wars used it, but the specific storyline in that film was unique. The movie Eragon's storyline, in contrast, is practically note by note identical to Star Wars. Replacing the lightsabers with swords and the spaceships with dragons and horses doesn't make the narrative different.
What makes this worse is that Eragon doesn't even retell that story well. A lot of its scenes just putter out and give up at the end, and you never feel like you know anything about the characters or their personalities. It doesn't even feel like you really saw a film by the time it's over. Believe it or not, I might have actually excused the movie if it had told that story better than Star Wars. At least then it could have argued a case for its derivative-seeming nature.
This is the way I feel about Avatar. I never really minded how unoriginal the story in that film was because honestly, I don't think the movies it supposedly ripped off were that good to begin with. Condemning Avatar for borrowing from movies like Pocahontas and FernGully: The Last Rainforest is kind of like condemning The Hangover for borrowing from Dude, Where's My Car? It's not exactly tainting the good names of any masterpieces. Avatar may be corny and as subtle as a brick to the face, but it's one of the better versions of the "ignorant-person-destroying-the-forest-until-he-bonds-with-the-natives-and-saves-it" story.
Getting back on topic, what should an aspiring writer do if they realize their story idea isn't so original? Before deciding to pull the plug on your idea, you should take some time to really examine what you've got. How much does your story remind you of that other one that already used your premise? Does your story offer something unique from that other one? Most importantly, do you as the writer see potential in your story? If you have enough confidence in your idea and if you can even devise a way to make it more distinct from other similar works, you should proceed with it. If it seems like a weaker retread of something else, I would suggest setting it aside for later and going back to the drawing board in the meantime.
And if by chance you don't discover those similar works until after your story's been published, then hopefully you made your story as good as you possibly could.