All you need to do in the formula tool is select "Add a new column", name it, and then in the formula area itself just type the name of the field you want to duplicate. So if you want to duplicate [Field1], you would Add a new column, name it something else (i.e. DuplicateField1, etc.), and then in the formula area, type [Field1]. You'll see the list of fields pop up as you start to type it. Hope that helps!