[GENERAL] How to insert record only if primary key does not exist

[Andrus]

Table is defined as

CREATE TABLE firma1.klient (
  kood character(12) NOT NULL DEFAULT 
nextval('firma1.klient_kood_seq'::regclass),
....
);

How to insert record to this table only if primary key does not exist ?
I tried code below but got strange error as shown in log file.
Why this error occurs ?

Andrus.

2012-06-30 15:51:04 EEST ERROR:  duplicate key value violates unique constraint 
"klient_pkey"
2012-06-30 15:51:04 EEST DETAIL:  Key (kood)=(20037       ) already exists.
2012-06-30 15:51:04 EEST STATEMENT:  insert into klient (
    kood,
    nimi,
    tanav,
    piirkond,
    postiindek
    )
    select ((E'20037')), ((E'Statoil Fuel & Retail')), ((E'')), ((E'10148 
nnn')),((E''))
    from klient 
    where not exists (select 1 from klient where kood =((E'20037')))