Steps to reproduce:

1. Install Zabbix proxy CentOS 7 package.
2. Load database schema as instructed in the Proxy section of 'Installation from Packages' in the Zabbix user manual.
3. Set up a few hosts to be monitored by the proxy on the main Zabbix server.

Result:

In the zabbix_proxy.log, the proxy server receives configuration data from the server and chokes:

17314:20210208:112448.304 received configuration data from server at "10.2.13.160", datalen 4767558
17314:20210208:112453.166 [Z3005] query failed: [0] PGRES_FATAL_ERROR:ERROR: duplicate key value violates unique constraint "hstgrp_pkey"
DETAIL: Key (groupid)=(5) already exists.
[insert into hstgrp (groupid) values (5),(5);
]
17314:20210208:112453.167 failed to update local proxy configuration copy: database error

This is not the first issue of duplicate keys, either. There appear to be several.