Okay so I have managed to filter the P/O Details 1 table down so it's not duplicated by using this:

AND(
{Qty Backordered} > 0,
OR(
{Doc Type Det} = "1",
{Doc Type Det} = "2"
))

The only problem I am having now is I want to add this line to the filter but it's not working. What am I doing wrong??

{Line Modified?} = "False"