In music, it has been said that there is no new song ever written, only rehashes of the old ones. The same can be said of literature, and pretty much all things in life. In
Ecclesiastes 1:8-10(NIV) the Teacher says:
8 All things are wearisome,
more than one can say.
The eye never has enough of seeing,
nor the ear its fill of hearing. 9 What has been will be again,
what has been done will be done again;
there is nothing new under the sun.
10 Is there anything of which one can say,
"Look! This is something new"?
It was here already, long ago;
it was here before our time.
Just write what is in your heart, knowing that it is yours and yours alone, and know that it isn't just possible that it will be similar to another's... its almost inevitable. The key point is that it is new to you, it is yours, and your reader is very very unlikely to ever read the other book that is out there somewhere that tells the same or similar story, so it will be new to them as well.
I agree with sunstruckdream, in that the more you compare yourself, the more you are influenced by the work you are comparing yours to, and the less original yours will be.
Finally, I want to say this: even with all of what I said above, there is still hope for a completely original piece of work. Christ is
the Creator, through him all things are created, therefor seek the creator for your inspiration, worship him with your gift of writing, and see how he births something new in your heart and mind.
Also, don't be afraid to throw out a few starts... I've thrown out 4 now on a book I've been writing for 7 years. Be encouraged in this: With Christ, all things are possible. He will guide you and lead you and show you the way to go.