It re-shuffles the whole list again. However, there are two partial remedies for that:

• Make your shuffle mode 'Least recently played'. In that case just played tunes will be strongly weighted 'down', towards end of the running order, and you will be unlikely to hear them again in a day or two.
• Before unshuffling set a bookmark. When you are done with the album and want to return to your shuffled list, 'goto' that bookmark. (See FAQ for bookmarks - in short, they store the complete running order and your position within it.)
  

Since I usually can't decide what I want to listen to and consequently shuffle the whole player, this isn't of much concern for me.