subreport underlay

I have a main report that displays sales orders for each customer.

I have a subreport that displays all the outstanding invoices for each customer.

I placed the subreport in group header 1a: customer id

I am using the underlay function so that the subreport displays in line with the main report's sales orders information.

However, I am noticing that customers that span across multiple pages reprint the subreport on each page and the data overlaps with other customer's outstanding invoices.

Any idea on how to correct this?

Regards,

Zack H.