Mmm, I would think that every cel would have had 
an existing douga/pencil sketch at one time. (It 
might be destroyed now but should have existed 
back during the production process.) If a pencil 
sketch isn't done because the image is exactly the 
same as before then a cel wouldn't be done 
either-- it would be a waste of time and 
resources. They would simply pan the camera over 
the one cel to create the movement.