Now, before I get hip-deep in this little discussion; I'm not in any way, shape, or form, trying to say there is nothing new to find or try. I believe the capacity for the human imagination truly is infinite and that we will never reach the boundaries of what we can come up with.
The truly original storyline or piece of art is fast becoming a holy grail. Let's be honest, there is sooo much out there now that to honestly believe you've written something that is nothing like anything that's been done before is either arrogance or ignorance. Just because you've never seen or read it before doesn't mean it's not out there. Hundreds, if not thousands of new books are published each year, on top of what's been put out since the printing press was first invented. Can anyone really claim to have been through all of them to give authenticity to the idea that their new story is completely original? I don't think so.
I think we reach too far for that golden ring which is true originality and miss a lot of chances that are right in front of us. How many manuscripts have gone unfinished or never published simply because they're "similar" to another work? Probably more than anyone would like to admit.
I put this to you though, how "similar" does a work have to be before it loses its own voice? Every person on the planet is different (unless you believe in doppelgangers) and no two people are going to write something the same way. Different things affect us differently and while one writer might focus on the fear a particular scene engenders, another writer might focus on the courage of the individual experiencing it. Word choices, grammar usage, all change from writer to writer despite the conveyor belts that are the english classes we're put on through for all our school years.
Just because two stories might boil down to the same idea, (for instance a small band of survivors trying not to die during a zombie apocalypse) the execution makes all the difference and creates two extremely different pieces. Now, just because these two works start with the same idea, does that mean neither one is original? I get the feeling that most people would look at this example and say "Yes, neither book is original if they share the same idea.". But, if you were to hand them two different books and ask them if they think those are original, they would also say "Yes, they're original because they're different books."
They may have the same ideas, similar characters, and parallel endings, but it's really the meat of the story, not the specifics which really denotes whether a book is original or not. After all, for all their similarities, nobody will ever argue that an apple is an apple, an orange is an orange, and neither one is the other.
So whether you're writing a book or putting paint to canvas, my suggestion would be to do what feels right to you and don't worry about how original it is. After all, anything you create honestly, with your own voice, will be something that nobody else will have ever created before or after, despite the similarities people might pick out.
Rimmer: "I'm a competitive man, Kryten. Always have been."
Kryten: "We're all very well aware of what you are, Sir."