When evaluating trigger expressions Zabbix server:

1. extracts functions used in trigger expressions
2. calculates function values
3. replaces functions with calculated values in trigger expressions

Zabbix functions are unique across the triggers, so even if the item, function and parameters are the same, the function is still calculated once per every trigger it is used in. However if the trigger timestamp matches the function value will be the same and it can be calculated once for all triggers.