The option to enforce Unique Values is one of the small changes in 2010 that makes a big impact. Many times with SharePoint 2007, I have wanted to enforce that values only exist one time in a list. With the release of SharePoint 2010, this feature will be available OOTB. When you configure a new column, the option to enforce unique values is listed as one of the additional column settings.
If a user tries to use that same value in the list item, an error will be displayed prompting them to select a new value.
It is good to note that this setting can only be configured if there aren't any existing duplicate values in the list. If you already have existing duplicate values, those would need to be removed before turning on this column setting.