Whenever you think you might have some duplicate entries in Excel, run a quick check with Excel 2007/2010 conditional formatting.
Step 1: Select the data
Step 2: Click Conditional Formatting–>Highlight Cell Rules–>Duplicate Values…
You will then have a chance to see the duplicates with the color you specify before you click OK.
Final result: Red color shading over top of the duplicates.
Tip: Right click the “Duplicate Values…” command and then left click “add to the quick access toolbar.”