One of the best feelings for any database developer is knowing that duplicate data simply can't sneak into your solution. You've got clean data such that when you search for a specific email address you know there's only one and it belongs to the right entity.
In the associated video and sample file you'll get all the details for how you can use FileMaker's Unique value validation option. You'll see how you can prevent FileMaker from showing it's annoying dialog box which uses the name of the field when a duplicate is entered. Additionally, you'll be able to execute your own script when the validation is triggered and do really cool things like locate and present the existing instance to the user.
It's helpful features like this that make using a database system that much more user friendly. And this is where you'll find all the details.
Click the title or link to this article to view the video.