duplicate key violate error

Hi,

I got duplicate key violate error in the db log for the following query:
INSERT INTO tab1 ( SELECT '1611576', '1187865' WHERE NOT EXISTS (        SELECT 1 FROM tab1         WHERE id='1611576' AND id2='1187865'    )    )

The error occured during production time.
But when I manually executed the query, it inserted one row with success and next time it inserted 0 rows.

\d tab1
id int
id2 int
primary key (id,id2)

So, any idea why the error occurred at production time.