① Select the corresponding range of related cells below, click the Data tab, and select the Effectivity command.
② in the Pop-up Data Validation dialog box, click the Drop-down triangle to the right of "Allow" and select "Customize".
③ enter "=countif (A:A,A2) =1" in the dialog box below the formula. (A:a represents the determination of all cells in column A, A2 is to determine that the selected cell, equal to 1, refers to the uniqueness of the selected cell, and the entire function is meant to set each number in the entire area to be unique).
④ switch to the Error Alert tab, click the Drop-down triangle to the right of style, in the pop-up Drop-down list to choose the way you want, we choose to "stop" and then the title of the following dialog box entered the "Error warning", "error message" input corresponding text content, here input "input number error," Please re-enter, and then click OK to complete.
⑤ when you select a range of cells, you can only enter duplicates, and when you accidentally enter a duplicate, an error alert will automatically pop up.