There is a duplication cheat for Crystal... I can't remember it well, but it's something along the lines of:
Take two pokemon and deposit them into any box EXCEPT box 6. You must do this manually, not using the "Move Pokemon" option.
Go to the "Change Box" option, and change to Box 6.
When it says "Saving... Dont" then cut the game off.
When you turn the game back on, the two pokemon should be both in your party and in the box you deposited them into.
If someone could verify this, I don't remember it perfectly. I do know for a fact that this glitch exists.
;249; Lugia34