Hvorfor duplikeres mit resultat, når jeg bruger underforespørgsel?

select A.userNum from users A where A.userNum NOT IN  (select A.userNum from users A, addressbook B, (select A.userNum from users A, addressbook B where B.userNum=1 and B.phone=A.phone) C where C.userNum=B.userNum and A.phone=B.phone) ;