From Chip Pearson's site comes this info.
In Excel97 and later versions, you can use the Data Validation feature to
prevent a user from entering a number (or text) in a cell, if that number
already exists in a list. This feature is not available in versions of Excel
before Excel97.
For example, suppose your worksheet will contain a list of ID numbers for
employees, in cells A1:A50, and you want to prevent the user from entering
duplicate numbers in the list. First, select the range A1:A50. Then, go to the
Data menu, and select the Validation menu item. This will display the Data
Validation dialog. From the "Allow" drop down box, select Custom, and enter the
following formula in the Formula box:
=COUNTIF($A$1:$A$50,A1)=1
For an entire column change to =COUNTIF($A:$A,A1)=1
Gord Dibben MS Excel MVP
