I guess it was created by the Import function.
You should try including the column [Date Imported] in your duplicates search. If you can sort them by import date, you can probably pretty easily find the ones imported later.
I don't have a BackUp that I could use without messing with the work I have done the last couple of weeks.
MC makes automatic backups every couple of days or so. You really should have an automatic backup you can use if necessary. Even if you don't, this is a good time to say that you should make a backup before you try doing any deletions from MC's Library. That way you can go back if you do anything you really didn't want to do.
How do I safely remove all the duplicated entries without deleting tracks that are already used in Playlists?
Do you have exactly 2 copies of everything that's duplicated? Do you care about which copy it keeps in terms of the import date? For example, if it picked half from the original import date and half from a newer import date would that be ok with you? If yes to all of these questions, I think I have a procedure that will work for you.
But, as long as you haven't added any of the newly imported copies to playlists, you really should just be able to sort by Date Imported and delete the newer ones. Assuming they all got imported at once. Actually, that makes me think that you should probably figure out why you have two copies first. Otherwise, the problem might come back right?
Brian.