When I was at that point, I placed the 'real key' in a known position in the inventory, made a copy, which appeared in an other position, gave that copy to the dk, then made another copy. I was able to make three copies before the mirror bust.
The sad thing is ... I didn't want any of the stuff in three of the rooms ... I was really just hoping for the xp.
But an interesting exercise. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />