How to generate a sub report correctly for this scenario?

I’m currently modifying this report. Regarding to this report there is a custom table called APPromaItemList in my instance which contains the sales order details along with the proforma invoice details. Bellow screenshot depicts the modifying invoice report.

000010 is the corresponding sales order number for the above invoice report number 000004. Please check the below screenshot.

By this sales order number 000010 I'm generating a sub report of this APProformaItemList table to obtain the ProForma Invoice number. But when generating this the same Proforma Invoice Number is printing 4 times as shown in the above screenshot. It is not what I expected. I need only to print it once. So to overcome this issue ,I grouped by using the Reference Number in the APProforma as shown in the below screenshot.

But it was not success. So, to overcome this issue please give me some guidance. Thank you.