Cloning Pokemon by Hoeon Buddy
Pokemon Diamond on SuperCheats.com
Cloning Pokemon v2.0

Let me guess, you have been waiting so long for this great guide to be up and running and posted with FAQ and everything.
It took me a while to type this guide but here it is and enjoy!

Versions:
v1.0: First Cloning Guide, Not working 100%.
v2.0: Fixed typos, Errors, Grammer, added the FAQ and the "Versions" section.
_________________________

Step 1
Go to to the GTS (Global Trading Station) at Jubilife City, left of the Pokemon School. and log in.

Step 2
Once your in the main menu of the Global Trading Station. Go to "Deposit". Offer the Pokemon you want to clone and make sure you offer for a pokemon that no one would trade for, Because if someone trades for the pokemon you are wanting to clone, the whole process is ruined and you will need to start over from the very start.

Step 3
This is the hard part, It is considered to be the main step of the process.

After depositing your Pokemon It will now say: "Checking GTS's Status...."
On the right of that message, a clock should be going around. On the eighth
Spin, simply turn the DS off.

It doesn't need to be exactly 8 spins, it can be 6-16 depending on your wireless connection or just plain luck.
Your goal is to turn off DS off 1 spot (1 second) before the loading is complete.

Step 4
Turn your game on. You should get a error message:
"Your file has been corrupted. The previous file will be loaded"

Don't worry. This is normal and if it hasn't happened, you have failed, If you have succeeded continue on to step 6, If you failed go to the GTS and take it out and start again from step 2.

Step 5
If your character is right in front of the counter, your process has succeeded. If when you turn your game back on and he walks out of the slide doors, you have still succeeded but there is a small chance that someone has traded for your pokemon. If you succeeded, you cloned your pokemon. If you failed you will need to take your pokemon out and start from step 2 again.

Step 6
If you have succeeded (happens 99% of the time), Log Out and SAVE.

Step 7
Have fun and happy cheating!

Optional Way For Action Replay DS Users
9206B31E 00001808
0206B320 FB3EF7FB
94000130 FFF30000
0206B320 46C046C0
D2000000 00000000

For now 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 wanna duplicate. Make sure you have at least one free slot. Now go in the 'Move Pokémon' menu, press A on the Pokemon you wanna 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 voilŕ : 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.

FAQ
Q. Why can't I log onto the GTS?
A. You either do not have a Nintendo Wi-Fi connection or you are not in range of your wireless HUB.

Q. How do I get a Nintendo Wi-Fi Connection?
A. When you turn your game on, it should have 3-4 options depending on how far you are throughout the game, press A on "Nintendo Wi-Fi Settings". To configure it here you must have an AOSS lamp that plugs into the top of your DS or a wireless Network HUB (Wireless Internet/Nintendo USB port Wireless Plug). Make sure you get correct details and you have done "Yes" for the forms that say Auto Detect.

Q. Why hasn't my pokemon been cloned?
A. You have failed and have either not got the timing correct or have not done the cheat correctly, withdraw your pokemon from the Summary area at the GTS and try again from step 1.

Special Thanks
Thank you ThePokemonMaster for supplying the Action Replay Code