The Import Wizard allows you to perform multiple passes or scans in order to push the cleanest data possible to Salesforce.
From Tab 6 you have the ability to select Rescan to go back to Tab 5. From Tab 5 you may select different filter criteria to scan your import file against to find duplicates in various ways.
On the first scan you will start with the tightest filter and on additional passes you will work your way through looser filters or different fields. Be sure to review and merge your duplicate results after each scan. By rescanning your file, you have the ability to find duplicates on OR logic.