delete commands fails silently to delete primary key

I have table in 8.1.4 which tracks users logged into db

CREATE TABLE "session"
(
workplace character(16) NOT NULL,
ipaddress character(20),
logintime character(28),
loggeduser character(10),
CONSTRAINT session_pkey PRIMARY KEY (workplace)
);

Commands executed at logon in same transaction are:

delete from session where workplace=E'LIIVA' ;
insert into session (workplace,ipaddress,logintime,loggeduser) values (
E'LIIVA' , inet_client_addr()::CHAR(14),
current_timestamp::CHAR(28),CURRENT_USER)

Sometimes (during locking contention or during heavy load) those commands
cause error:

2008-11-22 11:24:26 EET INSERT 1 47433335ERROR: duplicate key violates
unique constraint "session_pkey"
2008-11-22 11:24:26 EET INSERT 2 47433335STATEMENT: delete from session
where workplace=E'LIIVA' ;insert into session
(workplace,ipaddress,logintime,loggeduser) values ( E'LIIVA' ,
inet_client_addr()::CHAR(14), current_timestamp::CHAR(28),CURRENT_USER)

No other client can add 'LIIVA' primary key.
Any idea why this error occurs and how to fix ?

Andrus.