When Duplicate Search identifies a duplicate case, the system displays a user form that lists the current case and the matching cases. The system prompts the user to either select a duplicate case and resolve it, or to continue processing the current case. This standard flow action pyDuplicateSearchCases uses properties such as case status, creation date, and match score to identify each case. You can customize pyDuplicateSearchCases by adding property values that help users identify and resolve the correct duplicate
In order to remove the duplicates from the Value List you need to write your own custom activity to iterate the entries in the Value List and remove the duplicate entries from the value list using @baseclass.StringListDisplay activity.
or you can iterate the entries in Value list then try to copy the unique values in the new value list in the activity.
Hope this approach might help you to achieve your requirement.