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I have an excel spreadsheet - is it possible to have validation so that a column cannot contain a value that already exists in the column?

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@DaveParillo - aiming for the Taxonomist badge, are you ? :)) – Rook Oct 21 '09 at 20:22
up vote 5 down vote accepted

Yes, from this example ...

  1. Select a range of cells, for example, A2:A20.
  2. Select Data -> Data Validation (in Data Tools Group).
  3. In Data Validation dialog box, select Settings -> Allow -> Custom.
  4. In the Formula box, enter the following formula: =COUNTIF($A$2:$A$20,A2)=1
  5. Select the Error Alert tab.
  6. In the Title box, enter: Duplicate Entry.
  7. In the Error message box, enter: The value you entered already appears in the list above.
  8. Click OK.
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Perfect, thanks – Jason Jul 23 '09 at 22:48
Sure -- glad to help! – JP Alioto Jul 23 '09 at 22:49

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