Pokémon Scarlet and Violet Duplication Glitch Lets You Clone Shiny Pokémon - IGN

Austin John Plays believes the glitch could be caused by entering an open town, at which point he believes the Pokémon in the world outside are put in a suspended state, but not despawned.

Ordinarily, heading back out of the town would cause them to respawn, but by closing the game and re-opening with the save in town, the game redraws the same Pokémon from the previous instance.

Glitches and what IGN called a "grevously poor performance" have been at the forefront of conversations around Pokémon Scarlet and Violet since it launched last week.