What can I do when iTunes multiplies albums?
I often have this issue, and I can fix it by selecting all of the offending tracks (e.g. the entire album) using Option + click, right clicking and changing the "Album Artist" to something random (I usually use "asdf") and then doing to same thing to change it to the correct Album Artist. Doing this forces iTunes to re-index the album and put the parts together.

If this does not fix it, then look for any other data that should match between all the tracks but doesn't. iTunes is splitting the album because of one of the tags, and by setting the offending tag to the same on all of the songs it forces iTunes to put it together.