I can tell you how to do this is Open Office...I'm guessing it's very similar in Excel. You can use conditional formatting.
Highlight the cells you want to apply this to. Go to the Format menu on the top bar and select "Conditional Formatting."
Make the condition "Formula is" and fill in the field with "COUNTIF(A$1:A$11;A1)>1". Then, before you close the window, select the style you want the cell to be if there's a repeat- highlighted red, a box around the cell, a big X through the cell, etc. Then click "Okay".