Preemption means that someone has published on a topic with an analysis similar to yours, recently, before you, thereby precluding your article on the same topic. To check if your article has been preempted, you should look at the following:
Important rules to pay attention to while working on your seminar paper:
An always popular subject in constitutional law is the first amendment. Take a look at some of these resources dealing with first amendment issues to find a topic for your paper or article.