sdap: Skip exact duplicates when extending maps
    
    When extending map with entry that already
    exists in the map in the exacty same form,
    then there is no need to fail.
    
    We should only fail if we try to
    change purpose of already used sysdb
    attribute.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3120
    
    Signed-off-by: Lukas Slebodnik <lslebodn@redhat.com>
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>