Greg
one way is to use conditional formatting. Assume you want to check column
C. Select the whole column and choose Format | Conditional Formatting ...
Change the Condition to "Formula is" and enter: =COUNTIF(C:C,C1)>1 into the
box. Change the font or the background to a suitable highlight colour and
press OK.
Wherever/whenever you have a duplicate entry it will change colour
Regards
Trevor