Sometimes it's necessary to have an unique column with users, locations, etc. SharePoint don't offer this possibility and you have to develop one if you need this functionality.
Codeplex has created a policy for resolve the problem. You must install and deploy the UniqueColumnPolicy.wsp file. This feature is deployed as a farm-level feature and should be active when you deploy it. If you need to activate or deactivate the policy, go to Operations>>Manage Farm Features in the Central Administration site.
Once you have applied the policy to the column, if you try to insert a repeated user you must see a message like this: