A preemption check is the process for determining whether there is any pertinent legal or specialized literature on your topic that has been previously published. This is a crucial step in the process of writing a comment or a seminar paper, because you are supposed to be providing an original analysis. You don't want to simply rehash something someone else has already written.
Someone else writing about your general topic doesn't necessarily mean you are preempted. However, if someone has the same thesis or argument as you, your idea is preempted.
In addition to being preempted by another article, you can also be preempted by a case about your topic. For example, if you write about a circuit split and the Supreme Court were to rule on a related case before your article is published, you would be preempted.
Don't panic. Retool your idea. For example, you could change your thesis and argue for the other side of the issue. Another option could be to reframe the scope of your argument or your approach.
For example, let's say my working thesis statement is "Disney will likely begin lobbying for another copyright term extension act very soon, but they will be less successful because of how copyright policy has changed in light of the rise of the Internet."
As you'll see on the next page, during my preemption check I will ultimately find an article arguing that any future copyright term extension legislation is likely to be fundamentally different than those that occurred in the mid-1990's because of Golan v. Holder, a Supreme Court case about copyright term extensions in the context of international treaties. The scope of that thesis is very narrow, whereas mine is fairly broad (perhaps too broad, but I can tailor it further as I research-- as long as I stay away from the Golan v. Holder article's argument!).
I could also change my approach to the article instead of my scope. For example, another article I will eventually find in my preemption check compares copyright term extensions to patent laws, which is a unique approach.