ZBrushCentral

1 Model inside the main model. Can move, but as soon as i mask it "clones"

Hard to describe for the title, but here is my issue:

I made a model, and used the following tutorial to make the “mouthhole”
http://www.digitaltutors.com/11/training.php?vid=5670

I basically think i have missed an important step at the end and it seems the tutorial doesn’t show what you do to finish it up, since there is a totally different topic in the next video part.

I then proceeded to sculpt details, painted everything and so on, but then i tried to mask the arm, move it with the transpose tool and then the frustration began:

http://radioripper.webplace.at/move.jpg

Apart from the amateurish try of moving of the arm (i know…skelets, rigging and stuff…will look into that soon), what can i do?
Why can i move, sculpt and edit the model in any way I want, but as soon as i mask something off, i get a second model underneath?

Either that is a total mystery for you guys, or i am missing something simple.
Thanks in advance.