sequence problem - many rows

I have the following table
Column | Type | Modifiers
----------------+-------+-----------------------------------------------------------------
id | integer | not null default
nextval('cachedgroupmembers_id_seq'::regclass)
groupid | integer |
memberid | integer |
via | integer |
immediatepar | integer |
disabled | integer | not null default 0
Indexes:
"cachedgroupmembers_pkey" PRIMARY KEY, btree (id)
"cachedgroupmembers2" btree (memberid)
"cachedgroupmembers3" btree (groupid)
"disgroumem" btree (groupid, memberid, disabled)

And the sequence for the id column:
CREATE SEQUENCE cachedgroupmembers_id_seq CACHE 1000;

Now the table is empty and I try to insert a row in this table:
INSERT INTO cachedgroupmembers(groupid) values(55);
And it works.

Next I populate the table wittt aprox 700.000 records.
And I do again "INSERT INTO cachedgroupmembers(groupid) values(55);".
The error is : ERROR: duplicate key violates unique constraint
"cachedgroupmembers_pkey"

Thank you.