ERROR : 'tuple concurrently updated'

The following query is performed concurrently by two threads logged in
with two different users:

WITH raw_stat AS (
SELECT
host(client_addr) as client_addr,
pid ,
usename
FROM
pg_stat_activity
WHERE
usename = current_user
)
INSERT INTO my_stat(id, client_addr, pid, usename)
SELECT
nextval('mystat_sequence'), t.client_addr, t.pid, t.usename
FROM (
SELECT
client_addr, pid, usename
FROM
raw_stat s
WHERE
NOT EXISTS (
SELECT
NULL
FROM
my_stat u
WHERE
current_date = u.creation
AND
s.pid = u.pid
AND
s.client_addr = u.client_addr
AND
s.usename = u.usename
)
) t;

From time to time, I get the following error: "tuple concurrently updated"

I can't figure out what throw this error and why this error is thrown.
Can you shed a light ?

-------------------------------
Here is the sql definition of the table mystat.

**mystats.sql**

CREATE TABLE mystat
(
id bigint NOT NULL,
creation date NOT NULL DEFAULT current_date,

client_addr text NOT NULL,
pid integer NOT NULL,
usename name NOT NULL,
CONSTRAINT statistiques_connexions_pkey PRIMARY KEY (id)
)
WITH (
OIDS=FALSE
);