F_DUPUID
if (!(up->u_flags & F_DUPUID) && uid_exists((uid_t)up->u_uid)) {
if (!(up->u_flags & F_DUPUID) && user_exists(login_name)) {
if (!(up->u_flags & F_DUPUID) &&
u.u_flags |= F_DUPUID;
u.u_flags |= F_DUPUID;