There is a lovely little feature in SharePoint 2010 called “enforce unique values”. This prevents users from duplicating data. The option is available any time you create a column.
So when you have a list of companies for example, and you try to capture a new record with an existing company name, you will be stopped.
But! A common action in lists, is to restrict users from editing and viewing other items in that list. You can change the settings so that they can only edit / view their own items. (Found under List Settings – Advanced Settings).
If you try and activate this restriction while you have enforce unique values activated, you will get an error message :
If you read the error, it makes sense. What it is telling you, is that you can’t make users type unique values if they can’t see all the values available to them.
So if you want users to be restricted in what they see, you may get duplicate entries. If you don’t want duplicate entries, then all users must be able to see all the items in the list.
What you can do though, is give everyone read rights to the complete list and edit rights on their own. Then enforcing unique values will work just just fine.