Having never used the “Import From USB” command before I imagined that I would be asked which of the many many CD’s worth of music I wanted to load. So having sought out some missing albums on iTunes I pressed “Go” and now have more than 30Gb worth of duplicates whose names appear in Red on the screen alongside the originals.
I tried just deleting them and was pleased with the result, though it would take me many hours to delete all 30 Gb worth. But then I realised that in addition to the duplicate names I was deleting the original tracks. I cannot be the first idiot to have made this mistake, so I am not going to do any more until someone tells me how to do it.