Large infrastructure: >20 000 Zabbix proxies (of different 5.0.x versions, ~ 5.0.5-5.0.11, SQLite DB) spread all over a wide terrriory.
Connection between the central Zabbix server (5.0.11) and remote Zabbix proxies is unstable and weak.
Ocasionally a remote Zabbix proxy is being rebooted and after that Zabbix server gets duplicating metric values. Number of duplicates varies from several to hundreds.

The problem is being detected by observing increased history cache usage. There are examples when Zabbix proxy databse has a single entry whereas Zabbix server database has multiple duplicates of this entry.