Select E2:E10. E2 should be the active cell in the selection.
On the Home tab of the ribbon, click Conditional Formatting > New Rule...
Select 'Use a formula to determine which cells to format'.
Enter the formula
=COUNTIF($B$2:$B$100, E2)
Click Format...
Activate the Fill tab.
Select yellow as fill color.
Click OK, then click OK again.
Also see Hide Used Items in Drop Down List
