setfilter to display all the duplicate rows pb7

Friends,

my version is powerbuilder 7.

i am checking the duplicate rows in the datawindow before inserting.

in a button click event...i have the below code

dw_master.SetSort ("rollno A")

dw_master.Sort()

dw_master.SetFilter("rollno = rollno[-1]")

dw_master.Filter()

if dw_master.RowCount() > 0 then

    MessageBox('Duplicate Rows','Check the Duplicate Roll numbers',StopSign!,OK!)

    Return 1

end if

//no dups, so clear the filter and do the update...

dw_master.SetFilter ("")

dw_master.Filter()

i have 2 duplicate rows...but this setfilter is displaying only one row....

if it displays all the duplicate rows then it will be very useful for me...

how can i modify this code to get all the duplicate rows in this datawindow before inserting?

thanks