Service Portal List Collector's: How to differentiate between 2 selected users
1. One option you have would be to switch the configuration and set the "Email" or "username" field as the "Display Value" field for the sys_user table and include the "name" field in the attributes so that you can see the user's names in the dropdown. (PFA)
This will allow you to see the names and unique information in the dropdown, but have only the unique values populate the list collector so that you can differentiate values on populated lists.
2. Since the Name field on sys_user is a calculated field, the other option you have would be to update the script for the sys_dictionary record on the 'sys_user/name' field and include the user name or email in parentheses after the name so that the display value will then appear something like "John Smith (email@example.com)"
Changing the Name field on the sys_user table will impact how the sys_user.name field is displayed globally across the instance