One of my favorite features within Excel is to be able to delete duplicates from a list. I will write a post on that feature in the near future. Sometimes, however, you just want to know if there are duplicates. Not necessarily delete them.
The conditional formatting feature within Excel is the solution for this one.
Living under the HOME tab, the conditional formatting is pretty powerful and can provide all sorts of color coding for your needs. In this case, we choose Conditional Formatting, then Highlight Cells Rules, then Duplicate Values. You choose the color, and voila, you have instant recognition of duplicate values.