Upgrade our system to Tridion 2013 and Content Porter 2013 export is 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

Anyone seen this error before or know how to fix it?