How to mark duplicate values with color in Excel?
As we all know, there is a function called Remove Duplicates in Excel, which can help you quickly remove duplicate values from a list. However, in some cases, you do not need to remove duplicates but only want to mark the duplicate values with different color as below screenshot shown. In this time, how can you solve this job quickly in Excel?
Mark duplicates with color by Conditional Formatting
Identify and mark duplicates with color by Kutools for Excel 
Mark duplicates with color by Conditional Formatting
The Conditional Formatting feature can do you a nice favor on marking duplicates with color.
1. Select the list you want to mark the duplicate values from, click Home > Conditional Formatting > Highlight Cells Rules > Duplicate Values. See screenshot:
2. In the Duplicate Values dialog, select Duplicates from left drop down list, the go to right drop down list to select one format as you need to mark the duplicate values, you also can click Custom Format to choose the color as you need under Fill tab in the popped out Format Cells dialog. See screenshot:
3. Click OK, the duplicate values are marked with color.
Mark duplicates by formulas
If you just want to mark the duplicate values, you can apply a formula to make it outstanding.
Copy and paste one of below formulas to a blank cell, B2 for instance,
=COUNTIF($A$2:$A$12, A2)>1
=IF(COUNTIF($A$2:$A$12, $A2)>1, "Duplicate", "Unique")
Then drag auto fill handle down to copy the formula to the cells, if the cell displays TRUE or Duplicate, the relative cell is duplicate, or it is unique. Screenshot:
Identify and mark duplicates with color by Kutools for Excel
If you want to both select and mark duplicate values with color in Excel, there is no built-in function can help you. However, if you have Kutools for Excel – a multi-function add in tool, you can apply its Select Duplicate & Unique Cells utility to solve this job easily.
After installing Kutools for Excel, please do as below:(Free Download Kutools for Excel Now!)
1. Select the list you want to find duplicates and mark, click Kutools > Select > Select Duplicate & Unique Cells. See screenshot:
2. In then Select Duplicate & Unique Cells dialog, check Duplicates (Except 1st one) or All duplicates (Including 1st one) option as you need, then go to Processing of results section to specify the background or font color as you need. See screenshot:
3. Click Ok, and a dialog pops out to remind you how many duplicates have been selected.
4. Click OK to close it.
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