I took a look through Mandarake's web auction 
pages, and didn't see the sketch you referred 
to -- is it on one of their direct-sale sites?  
I'd be happy to take a look at both sketches and 
compare, if you'll email me URLs.  :)  I figure 
I've seen enough RX sketches at this point that 
I might be able to spot a fake.

In all honesty, though, I'd be more inclined to 
either agree with the previous poster, and say 
that the sketch owner was hedging his or her 
bets by offering it through more than one 
consignment dealer, or say that it might be a 
situation where extremely similar sketches were 
produced -- when you say "genga character design 
sketch", what exactly do you mean?  A settei 
sketch, with multiple views of the character, or 
a production genga, with art for a specific 
scene?  I have several genga/douga packets from 
various shows, RX included, where some of the 
sketches are nearly identical to each other.  
Setteis would be more problematic, since if they 
did more than one for a certain character, 
they'd be very different from each other, since 
the point is to show multiple angles and/or 
outfits.

I'm rambling.  *grin*  In short, send me more 
info, and I'll be happy to try to help!
  :)  C