Assume that you want to restrict the users from typing the duplicate values
in A Column.
Place the cursor in A1 cell and press Cntrl+Spacebar which will selecte the
A Column. Keep in mind that the active cell should be A1 cell. (Active cell
will have a white background after selection also)
Data >>Validation>>Settings>>Validation Criteria>>Allow>>Custom>> and in
Formula paste this =COUNTIF(A:A,A1)=1
Data>>Validation>>Error Alert>> Title enter this “Value Already Existâ€
Data>>Validation>>Error Alert>> Error Message enter this “Column already
consist this Value, So try some other valueâ€
Like this you can restrict the users by typing the duplicate value.
Remember to Click Yes, if this post helps!