If you deleted all products from your store and want to import them again you need to delete the products from Trash and re-import it again.
From your account click on "My products" menu
Select products you want to delete and click on Delete button
Now all Products will be deleted, to import again you need to visit the products catalog and select the products.
Note: Before starting re-import products from knawat make sure all products are deleted from your store.