I'm assuming that your Export Rule is pushing the file outside the Vault for processing by another business system? In that case, I believe that what you are seeing is just PDM dealing with the fact that Windows won't let you overwrite an existing file without some interaction from the user (try it). Typically in our Customers' implementations the XML file is processed and then deleted by whatever application is watching that folder, so this behavior wouldn't be a problem.
Would it work for you to have a unique name for each Export in stead of a random one, e.g. include the Assembly Name and Revision in the filename, or could your 'import' routine that processes the XML be modified to only process the highest number?
Customer Support Manager