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I use this on signup forms to prevent a user from registering a second account with the same email address.

CREATE Procedure sp_AddSubscriber
@Name as nvarchar(50),
@Email as nvarchar(50),
@AddSubscriber varchar(255) OUTPUT

AS
 IF (SELECT COUNT(Email)
                  FROM TSubscribers
                  WHERE Email = @Email) = 0
  
  BEGIN INSERT TSubscribers
  (Name, Email)  Values  (@Name, @Email)
SET @AddSubscriber = "True"                           
END              
ELSE
     SET @AddSubscriber = "The email address you have entered has already been registered in our database."
GO