failed insert into serial-type row

hi,

i'm working with an postgresql-installation on Solaris ("PostgreSQL
7.4.7 on sparc-sun-solaris2.9, compiled by GCC 2.95.3") and I have the
following problem:

I'm using tables like this:

--------------------------------

CREATE TABLE session.preferences

(

id serial NOT NULL,

name varchar(50),

value varchar(255),

CONSTRAINT session_preferences_pkey PRIMARY KEY (id)

)

WITHOUT OIDS;

ALTER TABLE session.preferences OWNER TO web;

--------------------------------

and sometimes if i'm adding new rows to the table with statements like
this:

insert into (name,value) ('xy','xy');

the database only returns an 'duplicate key on primary key', which
should be prevented by the serial-datatype, or? how could it be, that
two rows become the same id??

if i execute the insert-statement again it works.

the created serial-sequence looks like this:

--------------------------------

CREATE SEQUENCE session.preferences_id_seq

INCREMENT 1

MINVALUE 1

MAXVALUE 9223372036854775807

START 2487

CACHE 1;

ALTER TABLE session.preferences_id_seq OWNER TO web;

--------------------------------

what can I do against the duplicate-key problem? is it a configuration,
sql or operating-system specific problem?

with kind regards

stefan