--  Ok, given a simple table like this

CREATE TABLE Test2
(
    id INT IDENTITY(1,1)    --  ID column (simple customer number)
    ,fn VARCHAR(16)         --  First name
    ,ln VARCHAR(16)         --  Last name
    ,email VARCHAR(32)      --  email
)

--  With this test data
INSERT test2
SELECT 'gl', 'mi', 'gm@hotmail.com'
UNION ALL
SELECT 'a', 'b', 'dude@hotmail.com'
UNION ALL
--  Duplicate email
SELECT 'g', 'm', 'gm@hotmail.com'
UNION ALL
SELECT 'c', 'd', 'cd@hotmail.com'
UNION ALL
--  Duplicate email
SELECT 'e', 'f', 'cd@hotmail.com'
UNION ALL
--  Duplicate email
SELECT 'GG', 'MM', 'gm@hotmail.com'

--  This query will get the duplicated emails and their unique identifier (the ID)
--  You could add other columns to the mix to select
--  Note that the dude email does not show up, as it is not duplicated anywhere
SELECT DISTINCT a.id, a.email FROM Test2 a INNER JOIN Test2 b ON a.email = b.email
--  This line is the crux.  Basically, it says give me the rows between the two copies of the source
--  data, where the email is the ONLY thing that is the same.
WHERE a.fn <> b.fn AND a.ln <> b.ln
ORDER BY a.email