Very worried about this

I have a table that looks like this:

Table "public.hd"
Column | Type
| Modifiers
-------------------+-----------------------------+-------------------------------------------------------------------------
d_id | integer | not null default
nextval('public.hd_d_id_seq'::text)
h_id | integer | not null
src | text |
p_q | integer | not null default 1
c_id | integer | not null
insert_time | timestamp without time zone | default now()
Indexes:
"hd_pkey" PRIMARY KEY, btree (d_id)
"hd_idx" btree (h_id, c_id)

_Log output:_

[2005-03-24 15:33:25 EST - 13882 - dbname - 4052774] LOG: statement:
INSERT INTO hd (c_id, h_id, p_q) VALUES ((SELECT c_id FROM c WHERE e_id
= 'tester1'), (SELECT h_id FROM h JOIN c USING (c_id) WHERE e_id =
'tester1' AND h.active AND NOT p_c),10);

[2005-03-24 15:33:25 EST - 13882 - dbname - 4052774] ERROR: duplicate
key violates unique constraint "hd_pkey"

These inserts are being executed ever 1.5 seconds on this database.
This error does not happen often and appears to happen randomly. No
other inserts are being executed on this table except this one. From
our current tests 99% of the inserts go through, with the exception of
these few. Honestly, I don't see how this could be caused from
something on my end. The primary key value is being determined by a
default, so everything should be handled within the database. I'm using
postgres 8.0.1. Any ideas?

Thanks for the assist!

Kris