One of the things I really strive for with my writing is avoiding anything that's been done before. You can't avoid everything entirely. In some manner or another you'll be copying something, but get rid of as many "seen that already" problems in your manuscript as you can. This includes everything from cliche' sentences to predictable endings. If your audience can guess what can happen, it's anti-climactic when it actually does. Excite your readers by being original, and you'll be remembered a lot longer.