If I import the same product file twice, will I get duplicate products?

No. In Import / Export, products are matched by SKU, and categories, brands and tags by slug. A row whose SKU already exists updates that product; a new SKU creates a new product. On the mapping step, make sure the column marked match key is mapped. Every product row needs a SKU — rows without one are skipped and listed in the import errors, so fill in the SKUs before you import.