How to Select Every Row Where Column Value is NOT Distinct

This is significantly faster than the EXISTS way:

SELECT [EmailAddress], [CustomerName] FROM [Customers] WHERE [EmailAddress] IN
  (SELECT [EmailAddress] FROM [Customers] GROUP BY [EmailAddress] HAVING COUNT(*) > 1)

The thing that is incorrect with your query is that you are grouping by email and name, that forms a group of each unique set of email and name combined together and hence

aaron and [email protected]
christy and [email protected]
john and [email protected]

are treated as 3 different groups rather all belonging to 1 single group.

Please use the query as given below :

select emailaddress,customername from customers where emailaddress in
(select emailaddress from customers group by emailaddress having count(*) > 1)

select CustomerName,count(1) from Customers group by CustomerName having count(1) > 1