Thanks for this report, however I don't think that this is a bug. The -X option formats the output as XML. ExifTool does not have write support for XML files. Even without the duplicate LensFStops property, ExifTool will not update the date/time tags in the XML file with your 2nd command.
XMP is a specific type of XML. If you wanted XMP, then the command should look more like this: exiftool -o %d%f.xmp DSC_6108.NEF - Phil -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org