Why Pokémon BDSP's Duplication Glitch No Longer Works
Pokémon Brilliant Diamond and Shining Pearl had a duplication glitch that let players clone Pokémon and items, but it no longer works. The original games in the Pokémon series were notorious for their glitches, and it seems that Game Freak has fallen into old habits, as Pokémon Sword and Shield, as well as the Sinnoh remakes, launched with a number of nasty bugs. This is likely due to the games being able to add content via patches, which means that games can be shipped in an unpolished state, and fixed after launch.
Pokémon Brilliant Diamond and Shining Pearl have already received three patches since launch. The first patch added online features, the post-game content, and various cutscenes; the second update optimized parts of the game, and the third update patched a number of glitches, including some that could softlock the game. The patches fixed some glitches that allowed players to access content that is meant to be sealed off, including the hidden encounter with Shaymin.
The most notorious bug in Pokémon Brilliant Diamond and Shining Pearl was the duplication glitch. It was possible to use the Daycare Center in Solaceon Town to copy a Pokémon. It was also possible to copy any held-item that the Pokémon was holding. This allowed players to quickly clone the Legendary Pokémon, and anything else they needed or could be traded. The days of Pokémon cloning in Sinnoh have come to an end, however, as the duplication glitch no longer works.
The most recent patch for Pokémon BDSP has fixed the duplication glitch, as well as the Shaymin glitch. The people who have yet to update their game can still use the glitch, but they will need to update if they want to use the online features again, which limits its usage. The Pokémon and items that were already duplicated are fine and haven't been wiped, but no more can be duplicated without avoiding the latest update.
There was a time when the Pokémon games were released first in Japan, with international versions arriving later. One of the benefits of waiting was that the developers had the chance to fix any bugs in the Japanese version, such as the infamous door surfing glitch in Pokémon Diamond and Pearl. The simultaneous worldwide launches of the new Pokémon games means fans in Europe and North America don't have to wait as long, but they're also seeing the games in their rawest and most unpolished form.