create table dummy ( 
  id int auto_increment, 
  name varchar(255) not null,
  bla varchar(255) not null,
  primary key (id),
  unique key(name)
) ENGINE=InnoDB;

insert into dummy ( id, name, bla ) 
values ( NULL, "foo", "--" ) 
on duplicate key update bla = if (VALUES(bla) = "--", bla, VALUES(bla));

-- new row. correctly returns the new id (1)
select last_insert_id();

insert into dummy ( id, name, bla ) 
values ( NULL, "foo", "--" ) 
on duplicate key update bla = if (VALUES(bla) = "--", bla, VALUES(bla));

-- no update. correctly returns 0
select last_insert_id();

insert into dummy ( id, name, bla ) 
values ( NULL, "foo", "XX" ) 
on duplicate key update bla = if (VALUES(bla) = "--", bla, VALUES(bla));

-- row updated. THIS RETURNS 2! WTF??
select last_insert_id();

insert into dummy ( id, name, bla ) 
values ( NULL, "foo", "XX" ) 
on duplicate key update bla = if (values(bla) = "--", bla, VALUES(bla));

-- no update. correctly returns 0
select last_insert_id();