It is believed that the issue may arise from how iTunes names files, as while it typically adds a number to the end of duplicate filenames, it is not always doing this in the case of the SD videos. Workarounds appear to be available in two forms: the first involves enabling simultaneous downloads, which may force iTunes to generate separate names. The second entails pausing an SD download before it completes, and either restarting iTunes, or renaming and/or moving the HD file.