A super easy-to-do and fast item duplication bug exists now. It most likely has to do with the way followers are moved between shards. I posit that when the player is traveling from one shard to another with a follower, the code is creating a temporary variable to remember what the follower has, then deletes that follower in the first shard, then creates a copy of that follower in the second shard. This temporary variable is never cleared, so when the player travels from one world to another (i.e. SW to RoG) the code still sees that temporary variable and then creates a new copy of that follower, but when traveling between worlds it does not delete the previous follower therefore allowing for item duplication. After creating this temporary variable (with one trip to the caves or volcano) this will happen every single time you change worlds, you do not need to repeat the enter different shard step to repeatedly multiple items. Therefore, with just two loading screens you can double or triple (with chester) your items over and over again. I am fairly certain this is a temporary variable not being cleared problem because you can stop the duplication by save/exit/load and it will cease to duplicate.
Here is a video of the bug being discovered by accident, afterwards we went on to figure out why and how to make it more efficient (with chester) https://www.twitch.tv/videos/443526397
Take a follower (either chester or packim) into another shard (caves/volcano). Put items in them. Exit back to the main shard (RoG surface / SW surface), use the skyworthy or seaworthy to travel to a different world. With chester, in that new world you'll have a copy of chester with the same items in his inventory. Next, travel back to the original world, you will have two of your followers both with the same inventory doubling (or tripling in the case of chester) the original number of items you have. Extremely easy to reproduce or even to do by accident while playing the game normally.