» Validating Data To Be Unique In A Range

Problem:

Validiating the values entered in column A, so that a value could not be entered more than once.

Solution:

Select column A-->Data-->Validation-->choose Custom-->Enter this formula:
=COUNTIF($A:$A,$A1)<=1
Thus, when trying to enter ""1"" in cell A10, an error message will pop out,
and will prevent the user from entering any of the values already stored in column A.
Screenshot // Validating Data To Be Unique In A Range