Creating drop-down lists in Excel (2003 or 2007)

I'm using an excel list (or table in 2007), and I want one of the columns to have a drop-down list of whatever is in the column, or to be able to enter a value as text. I have tried using the data validation tool, but this produces a list full of duplicates (the column is for 'department' so not every entry will be unique). So basically if the table looked like this:

1       2
Name    Department
Jeff    IT
Bob     IT
Bill    English
Steve   Business
Dave    Business
Paul    Maths

Then the drop-down list for department would look like this:

IT

English

Business

Maths

Whereas with the validation tool it creates two entries for IT and Business, which would be no good.

Is there an easy way to do this?