On the server behaviors panel, there is a server behavior name "Server Validation" One of the Available validation types is Unique Database Value. This Server validation type is used to check that the email entered is unique email address.
if the Unique DB Value validation type is not working, send a copy of your page so I can see the code. the is_unique:true code in the spry validation is really just a place holder as a reminder to add the Unique DB Value server validation type, it will be ignored by the spry code.
Creating email to send the user to have them validate their email address is a technique called double opt-in. There is no tutorial for this, but the following thread has an explanation of how to create it: