I am so sorry but this is impossible.
No you cannot duplicate a ditto on Pokemon Diamond because they would be turning into themselves which is virtually impossible because you are already yourself so you would basically be doing nothing.
You get nothing.
nothing
nothing
Splash does nothing.
Nothing.
There no cheats* that can be activated in the game itself. A GameShark or ActionReplay will allow you to activate cheats. Ex: Max. PP, Max EP, Max. Stats, ect. In some of the first Pokemon games, Red and Crystal for a fact, have a glitch. In Red you can get 100 of anything, and in crystal you can duplicate Pokemon using a masterball. *(That I know of, I have looked on every cheat cite and have found nothing.)
nothing happens.
Nothing
theres nothing passed it
You don't. its for the plot. Nothing more
nothing will