IPA: process_members() add ghosts only once
    
    Since ghost entries might not be properly removed on the IPA server
    (https://fedorahosted.org/sssd/ticket/2567) chances are that during
    extdom group lookups a single user is returned multiple time. This patch
    removes the duplicates before trying to write the data to the cache.
    
    Related to https://fedorahosted.org/sssd/ticket/2159
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>