Insert Not Exists Dual PK Problem

Hi  I am trying to populate a table with 2 FKs as its PK (SiteID and ProductDescID).  First 1) I add in all the products whose Manfacturer and Category are supposed to appear on the site and then 2) I add in all the extra products that are needed regardless of their manufacturer or category.   The problem I am having is if the product has already been added to the ProductCatTable due to its Manufacturer or Cateogry but is also in the SatForceProduct table.  The can’t insert duplicate PK error is thrown.  I don’t know how to do this IF NOT EXISTS statement (or what ever else may be needed) so that I can check whether a line from the Forced table needs to be added.  I am not passing in any parameters and I am expecting more than 1 line to be inserted in each of the statements. Please help -- 1) Populate      INSERT INTO            dbo.ProductCatTable            (SiteID, ProductDescID)             SELECT                dbo.SatSite.SiteID, dbo.ProductDesc.ProductDescID      FROM        dbo.SatManu INNER JOIN        dbo.ProductDesc ON dbo.SatManu.Manu = dbo.ProductDesc.Manu INNER JOIN        dbo.SatCats ON dbo.ProductDesc.Cat = dbo.SatCats.Cat INNER JOIN        dbo.SatSite ON dbo.SatManu.SatID = dbo.SatSite.SiteID AND         dbo.SatCats.SatID = dbo.SatSite.SiteID        2) Add Force Ins  IF NOT EXISTS(SELECT SiteID, ProductDescID FROM ProductCatTable WHERE ????????) BEGIN       INSERT INTO                        dbo. ProductCatTable                                    (SiteID, ProductDescID)       SELECT            SiteID, ProductDescID      FROM         dbo.SatForceProduct        END            Thanks in advance J