Sum of Projected Total based on Unique #

Hello, I am working on a saved opportunity search.  I am grouping it by a custom field (named probability) and then summing the projected totals.  The drill down contains more fields from the opportunity record - very straightforward search.  However, I've also added a column for the sales order number (if there is one).  My problem is that if there are multiple sales orders tied to an opportunity then I have multiple rows (which I don't mind because then I get to see the various sales orders).  However, I only want to sum the projected total once per opportunity.  My