Today, 

"Foreach functions are supported only within Caltulated items as part of aggregate‐calculation formulas. They cannot be invoked in plain item keys or trigger expressions outside this context"

https://www.zabbix.com/documentation/current/en/manual/appendix/functions/aggregate/foreach 

 

However, to check for values that are duplicates across many items, tx MAC addresses or SNMP EngineID's, we need something that can search for item values across many (all) hosts.

last_foreach() seems like a perfect candidate for this.

We would need preprocessing to go through the data from the foreach function, but that part is trivial.