How about if you moved the email criteria to the outer select and added a DISTINCT?

insert into userprofile(userID, fieldID, value,)

select distinct userID, 1000, 'external'

from user

where not exists (

select userID

from userprofile

where userid = user.userid

and fieldID = '1000')

and (user.email LIKE '%@hotmail%' or

user.email LIKE '%@gmail%' or

user.email LIKE '%@googlemail%' or

user.email LIKE '%@yahoo%')