Duplicates in your CRM are rarely a good thing. Luckily, it is such an important problem, that there are many different companies out there working to help you solve this probelm. Duplicate Check is just one of those. As the most downloaded de-duping app on the AppExchange, Duplicate Check does the job by comparing objects at the point of entry and allowing you to delete or merge dupes. It even works across objects, so that you can avoid entering new leads if they are already contacts!
Duplicate Check offers the most complete and extensive duplicate prevention tools. An all-in-one native application.