DISTINCT Or GROUP BY Does Not Work. Help

I have have a column where there would be some duplicate records, but I have tried using the IN list, DISTINCT, and GROUP BY, and it does not give me distinct record set.

However DISTINCT and GROUP BY will work as long as my columns to display remain only one, but the minute I add more columns to my SELECT statement it does not make them distinct anymore.

Example 1 works, but Example 2 does NOT. In Example 2, I have added more columns otherwise it identical to Example 1. In Example 3, I was using the IN list, but the results of Example 3 is identical to Example 2 !!

EXAMPLE 1:
SELECT DISTINCT email_address AS Email
FROM email_address
WHERE Send = 'y' and (DATEDIFF(DAY, Sent, getdate()) > 30)

EXAMPLE 2:
SELECT distinct email_address AS Email, email_address_ID AS ID, Sent
FROM email_address
WHERE Send = 'y' and (DATEDIFF(DAY, Sent, getdate()) > 30)

EXAMPLE 3:
SELECT email_address_ID AS ID, email_address AS Email, Sent
FROM email_address
WHERE email_address IN (SELECT DISTINCT email_address FROM email_address
WHERE Send = 'y' and (DATEDIFF(DAY, Sent, getdate()) > 30))