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