Excel formula can not double working!?

I need to put a title without 2 bottle!
For instance, A1 = Bob, A2 = Sammy, A3 = Apple mackintosh, I is unable to insert once again Sammy with A4 or even other cell phone, can I do this formula with Excel

Use information validation.Select the entire column in that case use Custom>Formula
=COUNTIF(A:A NEW, A1)=1 in that case OK

Thank everyone.Have mentioned your review about better explanation regarding process involving answering, tend to consider that everyone knows what direction to go instinctively.

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