The most recent update for Animal Crossing: New Horizons appears to have patched the latest in a string of item duplication glitches. Though Nintendo's patch notes do not specifically admit to removing the glitch, players have begrudgingly reported that the glitch no longer works since the update rolled out.
Players were using the item duplication glitch to make infinite copies of one of the most valuable items in New Horizons: Nook Miles Tickets (NMTs). Using the in-game postal service, players could easily create duplicate items, including NMTs. NMTs featured heavily in the diverse and intense player economy that grew around coveted villagers and high turnip prices, but, with the arrival of the glitch, the value of the tickets dropped harshly. Now that the glitch has been patched out, the economy will likely turn back around and the value of NMTs will surely rise again.
Nintendo's patch notes for the latest update doesn't say anything specific about the item duplication glitch, but simply states that "issues to further ensure an enjoyable gaming experience" have been addressed. Since the patch came out on May 21, users have widely reported that the item duplication glitch that allowed them to duplicate Nook Miles Tickets is no longer worked, leading them to believe that it was removed with the update.
This isn't the first item duplication glitch to be found and patched out of Animal Crossing: New Horizons, nor will it probably be the last. Each glitch has been more useful than the last. The original item dupe glitch could only be used on items that could be placed inside players' homes, and involved manipulating furniture in certain ways to cause the item duplication. The next glitch, upon which the recent NMT glitch was built, used the in-game mail system, but was limited to duplicating only a few items that could be sent through the mail. Both of these glitches were patched out of the game fairly quickly.
Nintendo has a long history of removing cheaters and glitches from its games, and its response to the Animal Crossing: New Horizons item duplication glitches has proven no different. Nintendo has already promised frequent and steady updates to Animal Crossing: New Horizons, and patching any other glitches like the item duplication glitch will likely be a large part of that. Nintendo is staying incredibly vigilant when it comes to any glitches that go against the main theme of the Animal Crossing game - namely, waiting and working for rewards and items instead of making them appear as if by magic for quick bells. If another glitch happens to appear, Nintendo will no doubt be quick to catch it in an attempt to keep players more honest in their island improvements.
Source: Nintendo