You may have got the volume in a condition where they show up multiple times in the mount index. To clear this up, unmount all the volumes on the client and then use the unmount by index feature on the server to clear up the index. You may have to unmount each volume multiple times.
You simply highlight the volume, choose unmount by index, and choose the user it is stuck on. You should see the user un-highlight in the list on the right.