Since there have been Pokémon games, there have been cloning and duplicating issues. Nowadays, these issues can be corrected by the developer, as was the case with a previous cloning and duplication issue in Pokémon Brilliant Diamond and Brilliant Pearl. Another cloning and duplication issue was found in the latest update for BDSP, although it's tricky to exploit without knowing what you're doing.
Related: Pokémon Shining Diamond and Shining Pearl are the first major titles to seem like a pure filler - Practical Impressions
The method for this glitch comes courtesy of YouTuber Blaines. First, keep in mind that getting the timing of this glitch wrong can cause the game to freeze. Make sure you save your game before attempting this to avoid losing your progress. Since this glitch relies on sound signals, a pair of headphones is useful to have a pair of headphones. It is also worth noting that you will need access to the Fly HM to be able to fix this glitch. Have all the Pokémon you want to clone group together in one box and make sure you have a free box before trying.
- Stand outside any Pokémon Center with your Pokétch in the top right corner of the screen on the HM feature.
- Hover the cursor over the Fly HM.
- Press the R + A buttons together repeatedly until the Fly menu stays open.
- Your character can now move while the Fly screen is displayed. Press up to enter the Pokémon Center - listen closely to make sure you hear the sound effect of the doors opening.
- Press up again, then press right six times to go down the escalators.
- Press right again, then press left five times, then press down twice.
- You are now outside the Pokémon Center. Use the right analog stick to select any city you can fly to.
- Press A to fly to a city, wait about a quarter of a second, then press X. Another menu will appear while you are above the flight menu. Press A again to fly to your selected destination. Incorrect timing of this operation can cause the game to freeze.
- Once in a new city, enter any house. Make sure you are away from any NPCs. Use the right analog stick to navigate the menu and select the Pokémon option.
- Press R to enter your Pokémon boxes. Press A, then press X.
- Go back to the Pokémon menu button, press R and go to the box you want to clone.
- Press Y twice to select the entire box multiple times. Press A once they are all selected to collect them.
- Navigate left or right in an empty Pokémon box and move the selection. Press B twice to exit the extra menus.
- Select the "Check Summary" option on a Pokémon, then press B again to exit the menu and complete the process.