It means that your table has a field with the name _Song_ID_pk. If you open Manage | database | fields and double cilck this field's definition, you'll find that "unique values" has been specified on the validation tab.
Normally a primary key field (_pk means primary key probably), auto-enters a number each time a new record is created. It sounds like the value being entered is for one reason or another not unique. You'll need to check the settings on this field and make changes as appropriate to keep the error message from appearing when you create a new records. (Change the value that's auto-entered (likely) or remove the unique values validation rule(very unlikely).)