It sounds like you may need a report with a subsummary part in a list view rather than a portal.
if you store customers data in a table of its own then you should not have duplicate customer names.
Seems like there is more to this problem than the brief description.
My best guess is that you have something like:
And the same customer name in invoices can then appear many times.
As has been advised:
With a portal to Customers on your Dashboard layout, would limit the list to one row per customer and then fields in either customer (using aggregate functions) or Invoices (Summary fields) can report totals for a given customer.
It is possible to use portal filtering and a self join to drop out duplicates, but I wouldn't recommend that approach if my above guess work is at all accurate.
You might want to look into using virtual lists. We've done similar work by using SQL Execute to pull the required data and then use some additional script steps to show the data in a virtual list.