Let's say that your tables follow the standard data model for invoicing:
If you place a portal to LineItems on your customers layout, you'll get every item ordered every time it was ordered by a customer--this matches what you report getting here.
But if you put a portal to Products on your customers layout, you'll get every Products ordered without any duplicates. And you can set up that portal such that clicking a row in the portal selects that item for adding to your current order.
Brilliant, thank you! I knew it was something simple I was missing.