In this article, I will show you how to use data validation to prevent users from entering duplicate values ​​in Excel.

How to prevent duplicate values ​​in Excel

1. Select the range A2:A20.

2. On the Data tab, in the Data Tools group, click Data Validation.

3. In Allow, click Custom.

4. In Formula, enter the formula shown below and click OK.

Explain:

• The COUNTIF function takes two arguments. =COUNTIF ($A$2:$A$20, A2) counts the number of values ​​in the range A2:A20 that are equal to the value in cell A2.
• This value can only appear once (=1) because I don’t want duplicate entries.
• Because I selected the range A2:A20 before clicking Data Validation, Excel automatically copies the formula to other cells.

5. To check, select cell A3 and click Data Validation.

6. Enter a duplicate number.

Result:

To change the alert text, go to the Input Message and Error Alert tabs.

In addition, you can also view many other excel articles here.