false unique constraint error...for me

Hi all,
I don't know if it's a bug or not...but things are quite strange for me.
My problem comes from a unique constraint violation whereas tha data I try
to insert in my table are different (at least for me).
My database is encoded using SQL_ASCII, postgresql 7.4.8 on a Red Hat
Advanced Server v3 or 4)
I created a table :
CREATE TABLE trace_object (
object_id serial NOT NULL,
object_barcode character varying(15) NOT NULL,
object_barcode_128 character varying(25),
);
ALTER TABLE ONLY trace_object ADD CONSTRAINT pk_trace_object PRIMARY KEY
(object_id);
ALTER TABLE ONLY trace_object ADD CONSTRAINT trace_object_object_barcode_key
UNIQUE (object_barcode);
ALTER TABLE ONLY trace_object ADD CONSTRAINT unique_barcode128 UNIQUE
(object_barcode_128);

The column "object_barcode" contains human readable barcode (e.g.
AB28662097) and the column "object_barcode_128" contains the "crypted"
barcode readable by LASER scanners once printed with the corresponding font.
We can determine the object_barcode_128 content, applying a function on
object_barcode (you can find it here :
http://grandzebu.net/informatique/codbar/code128_PLpgSQL.asc, sorry the
comments are in french). Let's call this function text2code128().

If I do :
INSERT INTO trace_object (object_barcode, object_barcode_128) VALUES
('AB28662097', text2code128('AB28662097'));
INSERT INTO trace_object (object_barcode, object_barcode_128) VALUES
('AB28662098', text2code128('AB28662098'));

I get the error : ERROR: duplicate key violates unique constraint
"unique_barcode128"
But the string returned by text2code128('AB28662097') and
text2code128('AB28662098') are different!!!, i.e., respectively ÌABÇ<b4ÅÃÎ
and ÌABÇ<b4ÆÊÎ.

Why do I get an error here? I really don't understand...I get this error
using my cgi interface, phpPgAdmin and command line.

Some other things :
If I drop the unique constraint unique_barcode128, I can insert my previous
data. Then if the request is :
SELECT object_barcode_128 FROM trace_object WHERE object_barcode_128 =
(SELECT text2code128('AB28662098'))
the two rows are returned ('ÌABÇ<b4ÅÃÎ' and 'ÌABÇ<b4ÆÊÎ').

if my request is :
SELECT object_barcode_128 FROM trace_object WHERE object_barcode_128 LIKE
(SELECT text2code128('AB28662098'))
I get one row 'ÌABÇ<b4ÆÊÎ'

could you please help me understanding what happens....
I know that the unique index is created using B-TREE (CREATE UNIQUE INDEX
unique_barcode128 ON trace_object USING btree (object_barcode_128)). Is
there a way to have a look at the content of this index? Do you know how it
works and where I can find more information abour it?

thank you for your help
Gérald

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