I have this table in Tableau which is made of many relationships from various tables.

The main idea is to associate with As of Table to calculate the rolling value.

Here I want to get the rolling 6 Months so I wrote this expression:

ZN(SUM(IF [Month Diff] < 6 THEN [Flight Count] END))

However, I found what I got was the sum of the duplicated rows.

I also calculated the single month, which is accurate:

ZN(SUM([Flight Count]))

I don't get why if there is no [MonthDiff], Tableau can accurately get single row but once this field is involved, all rows are counted.

Is there anyone could suggest a solution to sum only the unique row? Thanks!

PS. I still need to use [Port] as filter so I have to use the Key_YearMonth_Port.