How can I delete duplicate songs from iTunes?

See Duplicate songs in iTunes - Apple Community. The Show Duplicate Items feature, with the Same Album option is useful, but doesn't make it clear that all copies of the repeated tracks are shown, and if you delete them all you'll have no original copy to play going forward. You need to be selective in what you delete.

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File > Library > Show Duplicate Items

You may need to go into Album view and select all the albums before doing the command.

It will list all the duplicates and then you can check the times and titles and decide what is truly a duplicate and what is not.

I believe there are apps that you can get to do this.