After upgrading our system to SDL Tridion 2013 and Content Porter 2013, content exports are failing immediately with the following error:

> System.Runtime.InteropServices.COMException (0x800A01C9): This key is already associated with an element of this collection|Error while getting virtual URL mappings table[Export failed at ExporterBL.ExporterClass.Export(String& Summary) at Tridion.Conte

Has anyone seen this error before and does anyone know how to fix it?