Duplicating items is how so many players obtain large amounts of gear or "kits" so quickly.
The most useful dupe (of its time) is the book dupe. The book dupe is done by saving the chunk with the items that you want to dupe, then taking the items out of the chunk, finally you put the dupe books in the chunk overloading it and causing it to revert back to the saved state with your items. For a easier to follow tutorial watch this video here.
A less common dupe but a dupe that still works is the donkey dupe. This dupe is accomplished by lagging the donkey out of the chunk and making them appear at the starting position. Instructions here.
The most common on 0b0t is the Crystal Dupe it involves players turning on auto disconnect while having a totem in their off hand, dropping the items the player wants to dupe on the ground, blowing up the crystal which will kick them and the logging back. To show new players how to dupe people will refer to this video here.
A more well known dupe is the Cactus dupe which involves players throwing their items on cactus (duping them) to dupe the items. One example of the cactus dupe on 0b0t here.