A law review/journal article or note must present original research. If another article has already been written on your topic with the same view you wish to propose, your topic has been preempted.
If you find any articles on your topic, but you will address the topic from a new perspective or provide a solution differing from that in the existing articles, you have not necessarily been preempted.
When is the preemption check conducted?
After you have chosen an article topic but before you begin your actual research, check for preempting articles. Also, check again before submitting your paper for publication.
Cast a wide net for articles, working papers, government publications, books, conferences, blogs, interest groups, and other sources which may discuss your topic. Also check for guidelines from the journal or review where you hope to be published.