It usually happens when we are importing data from a new MusicONE who’s exported the library meta-data from an automation system and the file has duplicate records; two copies of many tracks. On that first import, M1 doesn’t check for dupes because it has no way of knowing which copy of the audio file should be the one to use. After a song is imported in M1, subsequent imports will omit dupes; they won’t be imported if there is already a Song card in M1 with the exact same Title + Artist.
After the first bulk-data import, whenever a card is opened that has a duplicate with the same Title/Artist, it will alert you and not let you make any changes to the card unless the Title or Artist is changed to make it different, not identical to the other one. I’ll sometimes add a dot to the Title, which then makes the card unique, and that allows me to save and move the unneeded card to (deleted) songs.
If you should encounter a lot of duplicates, there is a way to find them all and then mass-delete them. When you do it this way, the tracks are flushed from M1 immediately, rather and moving to (deleted songs) or (deleted tracks), as with the manual deletes do.
