Sounds like you need to use the web admin to edit your attributes.
If you have a Performer attribute of "Nickelback" assigned to a file, and have a performer attribute of "Nikelback" (noticed how it is spelled differently) assigned to another file, then LMCE thinks that there are 2 different performers.
What I would do first, is go to Files&Media->Media Browser first, and filter all performers. Look through the list to see if there are any mis-spelled duplicates. If there is a duplicate, click on it to edit the attribute name, and change it to match the correct performer. When click finish, you will get a message that there is already an attribute with that name - and will give you the option to merge the two together.
Also, for advanced attribute editing, I recommend using the Files&Media->Media Files Sync page. However, don't run Media Files Sync from this page.. Just use it to go through your media, an drill down to its attributes. Then you can Add/Edit/Delete attributes on a per media file basis...