FileVault sparsefile uses double of actual allocated disk space
As I understand it, when you login, the encrypted disk image gets automatically mounted (and a link created to it). Thus, making your home folder /Users/moo point to the sparsebundle file within /Users/.moo. If you right click (or CTRL + click) on your home folder in the Finder and select Get Info, you should see thats its a Volume of a disk image with something like /Users/.moo/moo.sparsebundle. I beleive du reports its twice as its simply traversing the same effective folder structure again.