Pre-emption checking requires the potential author to compare their idea against others who have already published on similar topics. This does not mean the potential author is barred from writing on the same topic, but they must present their own unique argument. Keep in mind Volokh's recommendations for selecting a good topic: novel, nonobvious, useful and based on a sound legal analysis.
Ways to make differentiate your topic from similar papers that have already been published include arguments particularized to different jurisdictions, or groups. You can present a different set of facts or a different twist on the law. You should not be scare off because someone has already wrote on a topic which you are interested in writing on, but you must present your own ideas!
While the databases will overlap to some extent, you should still check each one since the contents and dates of coverage do vary. Follow these preemption check steps before you settle on your topic and repeat them with frequency until you submit your final draft.