Hiya!  I'm usually pretty quiet out here, but I 
think I can answer this question.  :)  Yes, 
production cels are one-of-a-kind.  For anything 
hand-drawn, it's pretty difficult to get exactly 
the same results twice.  What I mean is, each cel 
belongs to a sequence, and maybe the next in line 
only has one tiny difference between it and the 
first (maybe one lock of hair on a character has 
moved a millimeter to the right); but the 
difference is there.  If there are mistakes made 
and a cel isn't used in production, there still 
won't be another just like it.  It all depends on 
what kind of a scene the cels were used in.  If 
the scene is a still or pan shot, no other cels 
were made to complete a sequence.

I'm sorry if I sound like I'm beating this one to 
death for those who know this!  :)  Have a good 
one.