When I have songs associated with multiple projects, is there a trick to display different data in a project, such as lead singer? Or do I have to duplicate the song in order to do that?
To have different attachments (documents, recordings, MIDI presets), you can assign the song to both projects but assign the attachments only to the desired project.
To have different content in some fields, you can make a smart copy of the song and then assign the smart copy to a different project.
To have completely different content, you can make a regular copy of the song and then assign the copy to a different project.