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?
Yes, from this example ...
- Select a range of cells, for example, A2:A20.
- Select Data -> Data Validation (in Data Tools Group).
- In Data Validation dialog box, select Settings -> Allow -> Custom.
- In the Formula box, enter the following formula: =COUNTIF($A$2:$A$20,A2)=1
- Select the Error Alert tab.
- In the Title box, enter: Duplicate Entry.
- In the Error message box, enter: The value you entered already appears in the list above.
- Click OK.