Select a range of cells, for example, E2:E10.

From the Data menu, select Validation.

Select the Settings tab.

From the Allow dropdown list, select Custom.

In the Formula box, enter the following formula:=COUNTIF($E$2:$E$10,E2)=1

Select the Error Alert tab.

In the Error message box, enter "The value you entered already appears in the list above".

Click OK.