If you want duplicate values to stand out in an Excel spreadsheet, you can choose between two conditional formatting rules.

Boolean logic: The method used to evaluate conditions, returning either TRUE or FALSE. Logical functions: The primary logical ...

How to combine values into a single cell in .xls Excel files The newer features and functions that I’ll be using later in this article aren’t available in the earlier .xls format.

If you wish to count the same or duplicate values in a column only once in Excel or the count of occurances of duplicate values, read this article.