Force column unique

I want to set a rule for a column to force it to have unique values only.

That way if I fill in

It throws an error when I fill in the second bob.

Pretty much the same question I am trying to find an answer to. I am using Calc; I have a column of serial numbers; I want to make sure that I don’t mis-type something and enter two of the same number. How do I force the spreadsheet to disallow or warn me when I key in a value that is already present

Please, what are you talking about: Base, Writer, Calc? Try explaining with bit of detail the question.