For whatever reasons, copies of douga & gengas 
have been made, however for a different motive 
than you have suggested.  I have a few copied 
sketches but it's obvious from the colored 
pencil marks that they are still "original".  
Perhaps more than one person needed a copy of 
the same sketch?  I don't know what the 
reasoning for it would be, but I don't think ALL 
copied sketches come into being by greedy 
collectors.