Steps to reproduce:

1. Add a duplicate internal plugin in Zabbix agent 2
   1. easiest way just copy any register metric in any plugin init function

func init() {    plugin.RegisterMetrics(&impl, pluginName, metrics.List()...)    plugin.RegisterMetrics(&impl, pluginName, metrics.List()...)} 

1. Start Zabbix agent 2

Result:

Zabbix agent 2 starts with the duplicate plugin silently failing.

Expected:
Zabbix agent 2 used to fall in panic, currently only loadable plugins panic.