You may get duplicate listings, but you'll get a feel for which engine is providing you with the best matches for your query. If you want more listings from any one search engine, you can then visit the ones that have the most relevant data. Explore the sites you visit. If you find a single page article that has useful information on it, take the time to explore what else the site offers. Use the site's built-in search engine.
Many sites incorporate their own on-site search engine.
These are very useful for finding the specific information you might be after. This is useful on a page with a lot of content. Try to access the site at a later time. You can type in the domain name and it will return a screen that tells you if and to whom the domain name has been registered. Sometimes the owner of the file has changed services or reconstructed the site.