--rdf-update will append all data to the end of your database. It doesn't handle duplicates, but in the admin panel there is a "Duplicates" link letting you identify and remove duplicates from your database.
To import into a different root category, use:
--rdf-destination="Foo/Bar"
To move links from there use the category browser in the admin panel.
To import into a different root category, use:
--rdf-destination="Foo/Bar"
To move links from there use the category browser in the admin panel.