If you go to Posts > All Posts, you can change the number of posts displayed per page under Screen Options (top right corner of the screen, click on the error to expand this menu). Then, you can select all posts (by checking the box next to "Title") and use the Bulk Edit feature to delete all posts. You will need to repeat the process for Pages and Media Library.
When you run an import, posts/pages/comments are not duplicated - the importer recognizes what has been imported already.
If you need any help with your import in the future, don't hesitate to ask!