BUG #6393: cluster sometime fail under heavy concurrent write load

The following bug has been logged on the website:

Bug reference:      6393
Logged by:          Maxim Boguk
Email address:      maxim.boguk@gmail.com
PostgreSQL version: 9.0.6
Operating system:   Linux Ubuntu
Description:=20=20=20=20=20=20=20=20

I have heavy write-load table under PostgreSQL 9.0.6 and sometime (not
always but more then 50% chance) i'm getting the next error during cluster:

db=3D# cluster public.enqueued_mail;
ERROR:  duplicate key value violates unique constraint
"pg_toast_119685646_index"
DETAIL:  Key (chunk_id, chunk_seq)=3D(119685590, 0) already exists.

chunk_id different each time.

No uncommon datatypes exists in the table.

Currently I work on create reproducible test case (but it seems require 2-3
open write transaction on the table).