Will adding an existing item to a different shelf create duplicate, or move item?

[Karl]

I'm trying to figure out a way to communicate with GoodReads through my BlackBerry and use it to track what I'm currently reading and what I've finished reading (in the future, I may use it for a to-read list as well).

One of the vital components of this would be the ability to change the status of a book from "reading" to "read". I was thinking I would do this with shelves, but if I add, say "War and Peace" to my "read" shelf, and it's already on my "reading" shelf, what will happen?

[Michael]

assuming "reading" is "currently-reading" thats an exclusive shelf, adding a review to any exclusive shelf will remove it from the current exclusive shelf.