Pokémon Diamond (U) code
Duplicate Pokémon in PC
9206B31E 00001808
0206B320 FB3EF7FB
94000130 FFF30000
0206B320 46C046C0
D2000000 00000000
This code is a bit 'dangerous', meaning if you don't follow the instructions the game may crash. Enter the PC, and (if it's not already in there) put in a box the Pokémon you want to duplicate. Make sure you have at least one free slot. Now go in the 'Move Pokémon' menu, press A on the Pokémon you want to move, so the list of choice appear, and then put the cursor on 'move' (it should be there by default), press 'Select+Start' while pressing A to take the Pokémon, and while the Pokémon is 'in your hand', press L and then R (to refresh the box) and voilà : your Pokémon will be duplicated (it'll be in the box, and in the hand) (FYI, not refreshing the box and dropping the Pokémon somewhere and trying to take the duplicated Pokémon WILL crash the game). Now that the box has been refreshed you can drop the Pokémon anywhere, and repeat the process as much you want.
-credit goes to my one of my friends who e-mailed this to me.
EDIT: This code SHOULD work in Pearl also, I haven't personally tested this but my friend says it works.