I tried this...

if exists(select email from user where

email LIKE '%@hotmail%' or

email LIKE '%@gmail%' or

email LIKE '%@googlemail%' or

email LIKE '%@yahoo%')

insert into userprofile(userID, fieldID, value, primaryVal, levelID)

select userID, 5001, 'external', 0, NULL

from user ju

where not exists (select userID from userprofile where userID = ju.userID

and fieldID = '5001')

Although the script does not create a duplicate row, it update every other user in the table i.e. with an email address like @bbc.co.uk.