Conditional formatting - Highlight Duplicate Values This feature is mostly used to identify duplicate values in a range of cells or array, it also identify the any error of data of unique records like ID numbers, serial numbers etc . How to do this – Step 1 - First select the range of cells, then click on Highlight cell rules (conditional formatting) under Home tab Step 2 - Select rule type as “Duplicate Values” Step 3 - Then click on OK