No Original Ideas
The most disheartening news lecture. EVER.
BUT that doesn't mean that you can't have an thought (that may not be as original as you'd like) and put a truly unique spin on it. People are unique even if their thoughts aren't. If you can immerse yourself in the idea then it will end up being unique in the same way that you as a person are.
Characters (good well thought characters) take on a life of their own. They become three dimensional and as unique as a living and breathing person. Authors who truly see their characters, who feel them, know them inside and out, can tell you the exact way that they would respond to a situation, will always write unique stories because their characters are unique.
One of my BIGGEST pet peeves when reading books is stock characters. Flat. Perfect. Mary-Sue characters. Our individuality is what makes us human and characters who lack that individuality seem inhumanly perfect to readers. Like cyborgs. Programmed to be perfect. Perfect Is Boring! Seriously. Imagine dating someone who was honestly perfect. Sure it would be great at first, then it would grate at your nerves. It would drive you to the brink of insanity because their perfection would highlight all your imperfections.
I'm not just talking about our main characters either. A book can only be as strong as it's cast. Any character who had dialogue (even some who don't) need to be round, full of life. A bad guy isn't just bad. They weren't just born that way. What shaped them into the person that they are today? Ha ha, now my Psychology minor is coming out--don't let me get on my soap box!
In conclusion, if you find out that aspects of your ideas have already been done don't give up on it. Really. You can make it yours. Plot often follow patterns, but characters and they way they handle what's thrown at them are what make a story great.