Issue Summary

Arguably, today, there is no way to count unique occurrences of items (Strings, numbers, list items, etc..) without relying on an external system. Distinct (Jira smart values - lists) is close enough, but it removes duplicates and we would need a way to count them instead:

• https://support.atlassian.com/cloud-automation/docs/jira-smart-values-lists/

Java example:
https://stackoverflow.com/questions/12719998/how-to-count-unique-values-in-an-arraylist

Starting from Java 8 you can use Stream:

After you add the elements in your ArrayList:
 
long n = wordStore.stream().distinct().count();

It converts your ArrayList to a stream and then it counts only the distinct elements.

Shell example:

$ echo word1 word1 word2 word3 word1 word4 word2 word2 | xargs -n1 | sort | uniq -c
   3 word1
   3 word2
   1 word3
   1 word4

Usage scenarios

• Counting the occurrences of a given word in a ticket's description
• Counting and grouping the number of selected items in a list custom field that have something in common (a substring for example)
• Counting and grouping the number of selected Objects in an Assets custom field that holds multiple objects

Workaround

An external system. ScriptRunner for Jira might be an option (citation needed)