The new releases of Pokémon Brilliant Diamond and Shining Pearl have been out in less than a week and it looks like a duplication glitch has already been discovered allowing trainers to clone Pokémon and even any attached items like Master Balls.
And here are the steps to reproduce this glitch (via Nintendo Everything):
1. Place two Pokemon that are in the same position in different boxes (for example, in the lower right corner).
2. Take one of these Pokémon and put them in the battle crate.
3. Go to Solaceon Kindergarten, talk to the woman and select a Pokemon from another crate.
4. Before closing, click on “Mailbox List” in the lower left corner.
5. Press “Y” and swap the slots they are placed in, then place your Pokemon.
6. Open your crates again and go to the Battle Crate.
7. Take the Pokemon you are going to “delete” and insert it.
8. Take the Pokemon you want to clone and add it to the list.
9. Review the summary screen of the newly added Pokemon; when you return it will be cloned
If you nevertheless decide to complete this glitch, it is at your own peril and risk. And it’s likely that Pokémon and developer ILCA will release a patch for that as soon as possible.