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.