I didn't even realize it !  Here are two, and I 
have one more on the way, but no pic, sorry.  
Here's Piccolo training against himself from 
Dragonball Z, and also Tiara fighting herself in 
the Throne Of Yord from Shamanic Princess.

The third cel I have is from Mezzo Forte.  It is 
a cel of Mikura meeting her Robot Self.  
Hopefully it should come today!  WooHoo!