We’ll use data validation to prevent duplicate numbers in the Membership Number column. ... If you’re working with your own data, be sure to update the name of the Table and column.

Open the Data Validation table by following the steps provided in the above fix. Under the Settings tab, click on the Clear All button. Click OK to save the changes.

Excel’s data validation feature is underused because many users don’t realize how versatile it is, especially where dates are concerned. Dates seem to complicate things, but only in your head!