Faseeh is on the right track, but the formula as posted will highlight all items that occur only once. To highlight duplicates you want
=COUNTIF($A$1:$A1,A1)>1
Step by step:
Assuming your data is in column A and starts in row 1 (adjust to your real scenario):
- select column A
- On the Home ribbon click Conditional Formatting > New Rule > Use a formula to determine...
- Copy the formula above into the formula box (and adjust to your column and first row number). The $ signs are important.
- Select a format and OK all dialogs.
Yes, that's how I understand the question: Highlight duplicates, i.e. show me items that appear more than once. If an item appears three or four (or more) times, it will also be highlighted.