You can find duplicate values in an Excel Spreadsheet by:
  1. Select the range of cells you think might contain duplicates.
  2. On the Home tab, select Conditional Formatting > Highlight Cells Rules > Duplicate Values.
  3. In the Duplicate Values box, make sure the Duplicate is selected in the first drop-down box.
  4. Then select the format for the duplicate values in the other drop-down box.  Select OK.
Once this is done all the duplicate numbers or text fields will be highlighted.

If you do not already have a group exported to Excel you can use this KB Solution.