MySQL Query Providing Duplicate Result Set on INNER JOIN - HELP

[pci_dev]

I designed a site awhile back, bugged, proofed the whole nine and everything worked fine before we went live. After six months or so, after a server upgrade I think, I got a complaint from a user that there were some duplicate entries showing up in certian sections. After looking into the code I've pin pointed it to my SQL JOINS, but I can't figure out why all the sudden their not working anymore.

More importantly I can't figure out how to pull only one line per result set. I know why it's pulling double entries, one from each table in the join. I can't figure out what i'm doing wronge how do I remove these duplicates from the query.

Help is greatly appreciated...

SELECT
TABLEA.id,
TABLEA.mem_id,
TABLEA.individual_list,
TABLEA.type1,
TABLEA.type2,
TABLEA.type3,
TABLEA.type4,
TABLEA.type5,
TABLEA.type6,
TABLEB.member_number,
TABLEB.first_name,
TABLEB.last_name
FROM TABLEA
INNER JOIN member_directory_la
ON TABLEA.mem_id = TABLEB.member_number
WHERE
TABLEA.individual_list='1'
AND
TABLEA.type1='1'
ORDER BY last_name ASC

[pci_dev]

OK... So I figured it out by reading another post in this forum but i'm still kinda curious to why this additional code right before ORDER BY singles out the resultset. Can anyone explain the GROUP BY tag ?

SELECT
TABLEA.id,
TABLEA.mem_id,
TABLEA.individual_list,
TABLEA.type1,
TABLEA.type2,
TABLEA.type3,
TABLEA.type4,
TABLEA.type5,
TABLEA.type6,
TABLEB.member_number,
TABLEB.first_name,
TABLEB.last_name
FROM TABLEA
INNER JOIN member_directory_la
ON TABLEA.mem_id = TABLEB.member_number
WHERE
TABLEA.individual_list='1'
AND
TABLEA.type1='1'
GROUP BY last_name
ORDER BY last_name ASC