SharePoint: How to have a unique constraint in a list?

Question

In SQL its possible to have fields that cannot contain duplicate data.

How is this possible in SharePoint?

Lets say you have a list containing values like

ORANGES
APPLES 
PEARS

How do you ensure that the user cannot enter

ORANGES
APPLES 
PEARS
ORANGES

Thanks

Answer 1

This can't be done out of the box. But there are these community solutions:

• SPListConstraints (free but not open source)
• Unique Column Policy (open source, original solution by Scot Hillier)

Answer 2

Looks like this can be done in 2010

If you go to your list settings and click on the column there is the option "Enforce unique values"