Make plot and outline notes. An author strives to be fresh and original at all times, but in reality this is not possible. All literature owes something, in terms of style or content, to earlier fiction. There is no reason that your new novel can't show the recognizable influence of other writers. When you've finished recording the major plot points and outline notes, do some research on the Internet to see what turns up. Search under keywords taken from the plot and outline notes.
Take a break and proofread the text again. Honesty is your only resource. When you write, you may find yourself unconsciously imitating a passage in fiction that you have read and admired in the past. To help in detecting this, proofread your text a few days after writing, with a fresh eye for unconscious plagiarism. If something sounds overly familiar, check a few plot summary texts and input key phrases on the Internet.
Join an authors' group. Share your manuscript with other writers in the group. Someone will usually catch a borrowed plotline or text.
Check your work carefully using plagiarism software. Even if you plagiarize unintentionally, others may not care why it happened. Your good reputation is all you have. Go the extra mile and make sure that your work doesn't pirate that of other writers by using the free online plagiarism tests listed in Resources. Many colleges and universities also have online portals to input sample text. Take the time to test any passage that seems overly familiar, or that others have suggested may derive from other sources.