Is your paper's main claim original? If there is a text that approaches your topic in the same way, and if you do not know about and address this text, then this text will preempt your paper. (I.E. If a text has already said the same thing in the same way, then that text preempts your paper because your paper has nothing new to add.)
Being preempted makes your paper irrelevant not just because you have the same approach to your topic, but because by failing to address such a relevant earlier text you reveal that your paper was not researched well enough to find this earlier text and so lacks the legitimacy, and perhaps the credibility, to be taken seriously.
The preemption check comes after you find and develop a topic for your paper. Once you have decided on a particular way to approach the topic (E.G. if you have drafted a main claim), then you need to find out if any other text has approached your topic the same way.
While there are a variety of good ways to do a preemption check, each of them requires some plan to ensure that you search all likely sources where you could find a text on the same topic and with the same approach that you plan for your paper. A suggested way to do your preemption check is under the "Search These Resources" page on this guide.
If you find a text that says exactly what you were planning to say, then you must decide how to avoid having your paper preempted by this text. Many choices are possible, but below are two typical ones.
If you find a text that says exactly what you were planning to say, and you decide on a way to avoid preemption, then discuss the way that you plan to use to avoid preemption with your professor to be sure that your paper is not preempted. Your professor might determine that you did not avoid preemption and that you need to take other steps.