If statements and date differences for values on separate tables

Hi!

I have a table visual which shows The client, the total amount owed, the last payment date and the next installment due date.

The last payment date and the next installment due date are on different tables.

I am trying to write a measure that says if the next installment due date is is greater than the last payment date, to mark "due" in another column. I can get the value of the difference using

Pledge Chase = datediff(SELECTEDVALUE(Pledges[Instalment Date Due]),SELECTEDVALUE('Original Pledges'[Latest Payment Made]),DAY)

But if i turn this into an if statement, I get multiple many duplicate rows in the table. Not sure why!