Complicated one for a Monday morning...
Let's see...
This get's you all the cover versions, plus the original...
[Media Type]=[Audio] ~nodup=[Name],[artist] ~dup=[name] ~sort=[Name],[Artist]BUT... (isn't there always)
It does not, and cannot, take into account the two versions of "All Around The World" shown in the screenshot below:
Obviously, the Oasis track and the Lisa Stansfield track are seperate tracks in their own right and are not wanted in this list.
There really is no way to exclude them either, which brings me to the next potential problem...
A cover is one artists interpretation of anothers, original, work.
You say that you want just one version of each track in your list, but it strikes me, that if the purpose of the list is to show cover versions, then you wouldn't really want the original version to be in the list either, and as it stands, if there are two tracks, an original and a cover, you have a 50-50 shot of the whether you get the original or the cover in the final list.
If you understand that and don't mind, fine, otherwise, you are going to have to work out a way to tag covers, as covers, and then incorporate that into your search string.
Now, for the next part, I may have grabbed the wrong end of the stick, but my understanding is that you only want one track of all the duplicates listed, and at the moment at least, you don't mind if that just happens to be the original version.
Going about it by limiting the artist to just one of each in the final list means that if you have different tracks covered by the same artist, for instance, I have a Bryan Ferry album here that consists entirely of Dylan covers, if any one of these appear in the list, then any other artists work that Bryan Ferry may have covered would be excluded because you have stated "limit to just one artist"
If you use the [name] field instead, then you get one version of each track, bearing in mind that you could still have tracks that share the same name, but are not covers, and also, you could have the original track, not the cover, in the final list:
[Media Type]=[Audio] ~nodup=[Name],[artist] ~sort=[Name],[Artist] ~dup=[name] ~limit=-1,1,[name]Ever wish you hadn't started something?
-marko.