If you right-click in the Supersearch tab to open a file from Usenet that's already been downloaded onto the hard disk (whether by adding to queue or by previously opening it in the same way), it will fail to open.
If 'skip file if already downloaded' is selected, it won't redownload or open the existing file, otherwise it'll redownload with the x extension, but not open the file afterwards (although file association would be off with the different file extension anyway).