Migrating XML data with many-to-one relationships
I ended up not using the "plugin:migration" option because apparently it assumes one-to-one relationships. So instead, I added code to the 'author' plugin's prepareRow to rejected incoming duplicate authors. Then I added code to my 'book' plugin's prepareRow to retrieve the author entity ids from the database and add it to the book entity. I was hoping for a built-in solution in the migrate system but this will do.