I don't think this has anything to do with de-duplication.
It's likely referring to iTunes Match which is a paid service that allowed you to upload your own MP3 files to Apple who would then serve it to you alongside any purchased/streamed music.
I believe that by default (but able to be disabled) your songs would be "upgraded" to the master-quality equivalent in Apple Music if they were identical. But then over time artists have been replacing those song with different versions/mixes.
It's likely referring to iTunes Match which is a paid service that allowed you to upload your own MP3 files to Apple who would then serve it to you alongside any purchased/streamed music.
I believe that by default (but able to be disabled) your songs would be "upgraded" to the master-quality equivalent in Apple Music if they were identical. But then over time artists have been replacing those song with different versions/mixes.