Duplicate records in a table inspite of a primary key

Hi,
We are having problems resulting from users being able to create duplicate
records
in table.

Below is the description of the table
Column | Type | Modifiers
-----------------------+------------------------+-----------
user_id | character varying(128) | not null
person_email_required | numeric(5,0) |
wddx_data | text |
proxy_id | character varying(128) |
email_to | character varying(6) |
password | character varying(60) |
user_name | character varying(64) |
language_preference | character varying(20) |
enable_autosearch | character varying(6) |
country_id | character varying(16) |
Indexes:
"i_personalise" PRIMARY KEY, btree (user_id)
"i_personalise_proxy_id" btree (proxy_id)
"i_personalise_user_id" btree (user_id)

database version: PostgreSQL 8.0.1

i_personalise- index

conn type: Primary (constriant Type)
condeferrable: False
conkey: 1 (column which has constraint)

Could you please help me out in finding the reason for such dupicate records
existing

Could you pls help me out in finding the reson for duplicate records being
inserted.