To highlight duplicates in Google Sheets, you can use the built-in "Conditional Formatting" feature. Here's how:
1. Open the Google Sheets document that you want to highlight duplicates in.
2. Select the range of cells that you want to check for duplicates.
3. Go to the "Format" menu in the Google Sheets menu bar and choose "Conditional formatting."
4. In the "Conditional formatting" dialog box, choose "Custom formula is" under the "Format cells if" dropdown menu.
5. In the "Value or formula" field, enter a formula to identify the duplicate values, such as "=countif($A$1:$A$100,A1)>1" for a range of A1:A100. This formula will count the number of times the value in cell A1 appears in the range A1:A100, and will return true if that count is greater than 1 (i.e. there is a duplicate).
6. Choose a formatting option for the duplicates, such as highlighting the cells in red or changing the font color to red.
7. Click "Done" to close the dialog box and apply the formatting.
Google Sheets will then highlight any cells in the selected range that contain duplicate values, based on the formula you entered in step 5. You can customize the formula to suit your specific needs, such as checking for duplicates in multiple columns or only highlighting duplicates that meet certain criteria.