It depends where the current content came from. If it is user-generated everything’s fine.
If (at least parts of it) were imported from phpBB3 (or some other forum software) you’ll need to delete a few things from the Discourse database, otherwise the importer will skip some content. The following SQL should do the trick:
DELETE FROM category_custom_fields WHERE name = 'import_id';
DELETE FROM post_custom_fields WHERE name = 'import_id';
DELETE FROM user_custom_fields WHERE name IN ('import_id', 'import_username');
This should work without any manual intervention. The importer tries to find an existing user which has the same email address or username before it creates a new user. It will reuse the existing user if it finds a match.
Oh, and you shouldn’t try this with your production database. Please create a backup and test the import in your development environment or at least a copy of your Discourse instance.