multi-column primary key insert error ('duplicate' complaint)

I have the following table where I want to use two columns as the primary:

create table MOVIES (

id                      char(12)        unique references PRODUCTS,
volume_id               int2            not null default 1,
label_id                integer         references LABELS(id),
length                  int2            not null,

primary key(id, volume_id)
);


However when I try the following:

insert into movies values('589000901097','1','1','1');
insert into movies values('589000901097','2','1','1');

I get this error:

ERROR:  Cannot insert a duplicate key into unique index movies_id_key

I thought I had defined the primary key to span two columns ... the data
in the first two columns is not a duplicate...

What did I miss?

Thanks!

Jc