Studio ILCA inc and Nintendo have released the Pokemon Brilliant Diamond Update 1.1.2 (also on Pokemon Shining Pearl) patch which appears to address the recently discovered item duplication glitch.

Pokemon Brilliant Diamond Update 1.1.2 Patch Notes | Pokemon Shining Pearl Update 1.1.2

• Fixed some issues that would prevent the game from progressing under certain circumstances.
• Some issues have also been fixed for more pleasant gameplay.

These are the official patch notes provided by Nintendo, however, many users on Reddit have confirmed that this update fixes the menu glitch, which allowed users to duplicate their own items. For those who plan on using the glitch, it is advised that you turn off automatic game updates and to disconnects from the internet altogether on the Nintendo Switch. 

Source: Nintendo