If you contrast
(1) 나는 여자친구가 있어요. I have a girl friend.
(2) 나는 여자친구가 없지는 (없진) 않아요. I have a girl friend.
the latter is constructed using double negation, '없지는' and '않아요'.
What is the difference between the two and when do you use No. 2 in place of No. 1?