I have two albums named "The Ultimate Collection" from two artists each in its own folder. Each album is on two discs (each in its sub folder) and properly tagged (i.e. Disc#1 and Disc#2).

When I set "Treat multiple disc sets as multiple albums"
I get 4 albums - 2 artists each with album on two CDs, each is treated as one album. This is OK.

When I set "Treat multiple disc sets as a single album"
I get 1 album - two artists with one "joined" album. This is NOT OK. There should be two albums, for each artist one.

If I rename one album to "The Ultimate Collection1" and rescan DB I get two albums as expected.

It seems that there is a bug which occurs when there are independent albums from different artists that have the same name and are on many CDs and SlimServer is set to "Treat multiple disc sets as a single album".

I would like to suggest to you to try to simulate this with the following folder structure:

MUSIC
  ARTIST1
    The Ultimate Collection
      CD1
        songs
      CD2
        songs
  ARTIST2
    The Ultimate Collection
      CD1
        songs
      CD2
        songs

and with setting "Treat multiple disc sets as a single album" and, of course, files properly tagged (i.e. Disc#1 and Disc#2). It should be enough to do the test with 4 songs, in each CD folder one.
I hope this will reproduce the bug.