Can you post a screenshot to show what you're referring to?
I am also looking for answer. do you got the answer, if yes please post the approch.
Thanks in advance
Tableau is a reporting tool, not a data manipulation tool per se. However if you need any calculations done without repeating the duplicates, you can always use the COUNTD function, it virtually eliminates the duplicates.
You can always post a packaged workbook if you need any help.