ERROR: duplicate key violates unique constraint "pg_class_oid_index"

ERROR: duplicate key violates unique constraint "pg_class_oid_index"

I found a post earlier but wanted to post up my info as the other persons did not look like the same type of error:

I tried to cluster the isbn_table

cluster isbn_index_code on isbn_table;
ERROR: duplicate key violates unique constraint "pg_class_oid_index"

The table looks like this

Table "public.isbn_table"
Column | Type | Modifiers
---------+--------+-----------
item_id | bigint | not null
date | date |
code | isbn |
Indexes:
"isbn_pkey_item_id" primary key, btree (item_id)
"isbn_index_code" btree (code)
"isbn_index_date" btree (date)
"isbn_index_date_code" btree (date, code)
Triggers:
checkisbn BEFORE INSERT OR UPDATE ON isbn_table FOR EACH ROW EXECUTE PROCEDURE isbntrigger()

Here is the vacuum verbose of pg_class

INFO: vacuuming "pg_catalog.pg_class"
INFO: index "pg_class_oid_index" now contains 698 row versions in 7 pages
DETAIL: 168 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: index "pg_class_relname_nsp_index" now contains 698 row versions in 23 pages
DETAIL: 168 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "pg_class": removed 170 row versions in 10 pages
DETAIL: CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "pg_class": found 170 removable, 698 nonremovable row versions in 143 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 5717 unused item pointers.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
VACUUM

select * from pg_class where relname='isbn_table';

relname | relnamespace | reltype | relowner | relam | relfilenode | relpages | reltuples | reltoastrelid | reltoastidxid | relhasindex | relisshared | relkind | relnatts | relchecks | reltriggers | relukeys | relfkeys | relrefs | relhasoids | relhaspkey | relhasrules | relhassubclass | relacl
------------+--------------+------------+----------+-------+-------------+----------+-------------+---------------+---------------+-------------+-------------+---------+----------+-----------+-------------+----------+----------+---------+------------+------------+-------------+----------------+--------
isbn_table | 2200 | 4280007913 | 1 | 0 | 4281669172 | 13950 | 2.03181e+06 | 0 | 0 | t | f | r | 3 | 0 | 1 | 0 | 0 | 0 | f | t | f | f |
(1 row)

select oid,ctid,cmax,xmax,cmin,xmin,* from pg_class
where relname='isbn_table';

oid | ctid | cmax | xmax | cmin | xmin | relname | relnamespace | reltype | relowner | relam | relfilenode | relpages | reltuples | reltoastrelid | reltoastidxid | relhasindex | relisshared | relkind | relnatts | relchecks | reltriggers | relukeys | relfkeys | relrefs | relhasoids | relhaspkey | relhasrules | relhassubclass | relacl
------------+----------+------+------+------+----------+------------+--------------+------------+----------+-------+-------------+----------+-------------+---------------+---------------+-------------+-------------+---------+----------+-----------+-------------+----------+----------+---------+------------+------------+-------------+----------------+--------
4280007912 | (132,16) | 3 | 0 | 0 | 46981456 | isbn_table | 2200 | 4280007913 | 1 | 0 | 4281669172 | 13950 | 2.03181e+06 | 0 | 0 | t | f | r | 3 | 0 | 1 | 0 | 0 | 0 | f | t | f | f |
(1 row)