Nintendo has rolled out a short update for Animal Crossing New Horizons. Its purpose was to correct a serious bug affecting the balance of the game. The bug was the item duplication glitch which required two players in the local cooperative multiplayer mode of the game. By turning the table with a Nintendo Switch (for example) , if one of the players picked it up while it was spinning, it would be duplicated. This is no longer possible.
If the update was not done by itself, you can force your console to do it by pressing the + button on the icon of the Animal Crossing New Horizons home screen.