Check for Duplicates in an Excel List
To check an Exel list for duplicate entries, first select the list and name it using Insert, Name, Define and call it ListToCheck.

Now, select the list (and, assuming the list starts in cell A2), choose Format, Conditional formatting and type Formula Is and then
=Countif(ListToCheck,A2)>1

Then click Format to apply a format and click Ok.

Now each cell containing a duplicate entry will be formatted so you'll see it clearly.