Once you have done the import, if you wish to redo it, simply drop all of the tables from the database and import the clean master backup.
Then delete the attachments and avatar directories from the data directory and the attachments directory from the internal_data directory.
If you have installed any add-ons and want to remove them, simply delete the files from the server.
Your installation will then be as it was prior to importing and the import process can be started again.
If you do not perform the tasks above and do another import, you will end up with duplicate content: forums, threads, posts, avatars, and attachments.