There are multiple users with the same display name

Hi,

We have a user and when she get an item assigned to her she sees the following alert:

"There are multiple users with the same display name USERNAME and at least one of them does not have read permissions to some of the files"

Now I looked in the database and when I run the following query with the username:

 SELECT     
     [ProviderDisplayName]  
    ,[DisplayName]  
    ,[HasDisplayName]  
    ,[Domain]  
    ,[AccountName]  
    ,[UniqueUserId]  
    ,[LastSync]  
  FROM [Tfs_Configuration].[dbo].[tbl_Identity] where displayname like '%USERNAME%'  

Then I get 2 same usernames back, How can I get rid of one of them ? When I access TFS trough the portal I only find 1 occurence of this user.

We use VS2013 and TFS2013 update 4

Best regards