Suppose your data is in range A2:A100 and you want to highlight all those cells which are duplicates.
1. Select A2:A100 and Home Tab > Conditional Formatting > New Rule
2. Put following formula after clicking "Use a formula to determine which cells to format"
=COUNTIF($A$2:$A$100,A2)>1
3, Click on Format Button to format the cells accordingly.