Bug with sequence ?

First just let you know i solved my past issue, i used IPCHAINS to redirect port 80 to a non root port, look like there is some ressource system leaking in IPCHAINS, i moved from it to IPTABLE and all works fine now.

Back to postgres :)

let's say i have one table with the following schema:

Table test(num int not null primary key,.........);
i have a sequence to handle the num: create sequence test_num_seq;

When i insert a new tuple i use: insert into test values(nextval('test_num_seq'),......);

When i look at my postgres log, time to time i see:
2002-02-04 08:11:47 ERROR: Cannot insert a duplicate key into unique index test_pkey

It's really rare (considering the traffic i handle) but it shouldnt happen at all.

Ha using 7.2RC2 btw
Best regards