Problem described also in Zabbix forum - https://www.zabbix.com/forum/zabbix-help/450318-error-with-duplicate-values-on-tables-history_uint-columns-itemid-clock-ns .

Steps to reproduce:

1. Perform upgrade on Zabbix server from 5.0 LTS to 6.0 LTS. Database engine is PostgreSQL.
2. Execute SQL script "history_pk_prepare.sql".
3. Reinsert data from table "history_uint_old" to table "history_uint", who has new multi column primary key "history_uint_pkey". 
4. Problem also occurs when new data gets added from Zabbix Agent active item.
5. To find duplicate values:

   SELECT itemid, clock, ns, COUNT(*) FROM history_uint GROUP BY itemid, clock, ns HAVING COUNT(*) > 1 ORDER BY COUNT desc;

6. Item who receives lot of data from host. That generates duplicate values:

   logrt[C:\app\file.log,"Query speed:"]

   Item should receive from Zabbix Agent Active at least 100 values per second. So CPU can process at least 2 item values on same one nanosecond.
   Possibly item "log" also could reproduce issue.

Result:

INSERT INTO history_uint SELECT * FROM history_uint_old;
ERROR:  duplicate key value violates unique constraint "history_uint_pkey"
DETAIL:  Key (itemid, clock, ns)=(1442, 1657527245, 687061700) already exists.

Expected:
Successfully import all data from tables "history*_old" without indexes into newly created tables "history*" with indexes. 

Successfully adding new data from Zabbix Agent Active items to table "history_uint", also items whose data gets processed by CPU on same nano second.