Select the range of cells you want to check
Click Format
Click Conditional formatting
Under “Format cells if,” choose Custom formula is
Enter a duplicate-check formula such as `=COUNTIF(A:A,A1)>1`
Choose a formatting style, such as a fill color
Click Done
To highlight duplicates across multiple columns, adjust the formula to match your selected range
To highlight duplicates in a specific column, replace `A:A` and `A1` with the correct column and first cell in your range
To remove the highlighting, delete the conditional formatting rule