"duplicate key value violates unique constraint" during updating non unique field - postgresql

Question

We are trying to move our application to new postgresql cluster.
during doing that we noticed that application threw exception like that:
[2017-06-02 14:43:34,530] ........ (psycopg2.IntegrityError) duplicate key value violates unique constraint "items_url"
DETAIL: Key (url)=(http://www.domainname.ru/ap_module/content/article/400-professional/140-professional/11880) already exists.
[SQL: 'UPDATE items SET status=%(status)s WHERE items.id IN ....
it's very strange because:
the application writes to items fields, not items_url. items_url is indexes by items, actually
UPDATE only changes status fields that hasn't flag unique and also it is not a primary key.
table items:
id | integer | not null default nextval(('public.items_id_seq'::text)::regclass)
ctime | timestamp without time zone | not null default now()
pubdate | timestamp without time zone | not null default now()
resource_id | integer | not null default 0
url | text |
title | text |
description | text |
body | text |
status | smallint | not null default 0
image | text |
orig_id | integer | not null default 0
mtime | timestamp without time zone | not null default now()
checksum | text |
video_url | text |
audio_url | text |
content_type | smallint | default 0
author | text |
video | text |
fulltext_status | smallint | default 0
summary | text |
image_id | integer |
video_id | integer |
priority | smallint |
Indexes:
"items_pkey" PRIMARY KEY, btree (id)
"items_url" UNIQUE, btree (url)
"items_resource_id" btree (resource_id)
"ndx__items__ctime" btree (ctime)
"ndx__items__image" btree (image_id)
"ndx__items__mtime" btree (mtime)
"ndx__items__pubdate" btree (pubdate)
"ndx__items__video" btree (video_id)
Foreign-key constraints:
"items_fkey1" FOREIGN KEY (image_id) REFERENCES images(id) ON UPDATE CASCADE ON DELETE SET NULL
"items_fkey2" FOREIGN KEY (video_id) REFERENCES videos(id) ON UPDATE CASCADE ON DELETE SET NUL
Well, the question is why it happens and how can I troubleshoot this?
Thank you.
UPD1:
I tried to reproduce it on 9.4. - reproduced
Played with client_encoding. Encoding everywhere is the same.