There’s no built-in duplicate rule for Excel’s Data Validation feature, but you can combine the feature with the COUNTIF () function to get the job done. To do so competently, you need to know ...

Go to Data on the Ribbon, and in the Data Tools group, choose Data Validation. The Data Validation window that pops up allows you to set your criteria in many ways. For our example, choose Whole ...

Figure A We’ll add a data validation control to this sheet. Click the Data tab, and then, click Data Validation in the Data Tools group. In the resulting dialog, choose List from the Allow dropdown.