Two entries with the same primary key

Hello,

More then month ago we upgrade DB from 9.0 to 9.1 with pg_upgrade. Then we
move DB to another server with standard pg streaming replication.

Now we have two entries with the same primary key. And I do not know what
to do.

SELECT ctid, id from billing_invoices where id = 27362891;
ctid | id
--------------+----------
(1112690,11) | 27362891
(1112438,26) | 27362891

\d billing_invoices
Table "public.billing_invoices"
Column | Type |
Modifiers
----------------+-----------------------------+---------------------------------------------------------------
id | integer | not null default
nextval('billing_invoices_id_seq'::regclass)
...
created_at | timestamp without time zone |
updated_at | timestamp without time zone |
Indexes:
"billing_invoices_pkey" PRIMARY KEY, btree (id)