It's possible, but very risky. People have been kicked off Nintendo WFC for doing it.
To do it:
1. Go to the GTS
2. Offer any pokemon for trade, and note how many times the clock goes around before the pokemon goes into the pokeball and is put up
3. Take that pokemon back
4. Offer the pokemon you want to clone, for anything
5. exactly 1 turn before the pokemon is put up (within a quarter turn) turn off the DS
6. There will be one in the box that it was in before you put it up for trade (or your party, if that's where it was) and one on the GTS