Minecraft wiki entry for Block_and_item_duplication, associated text: Duplicate your most valuable gear: if you lose them to lava or the Void, you can just pull a backup from your backup chest! More specifically, you make a duplicate from your backup, so you can go through the entire process if you lose them again.
Duplicate Enchanted Books / Diamonds / Ancient Debris / Netherite Ingots / Emeralds: Once you have one, you'll have an infinite amount by duplicating it. Again, it is important to leave some stock as backup; you never know if you need more.
Duplicate Enchanted tools / Blocks: You'll never have to worry about running out of your tool / block of choice, since you can just duplicate them as well. Duplicate in bulk for items/blocks that will be used in large quantities.
Duplicate Shulker Boxes (with items inside): theoretically you can duplicate both the shulker box and the items it contains, since that info is stored within the shulker box itself. Use this to duplicate blocks in excessive quantities.
Duplicate Enchanted Equipment: combined with a grindstone, you can farm an infinite amount of EXP by removing enchantments from duplicated enchanted gear. While a decent, safe and consistent method of getting exp, building an Enderman Farm is faster.
Infiltrate Pay to Win servers (Not Endorsed): Servers that violate the EULA by selling in-game items for real money can be duped and sold. Duping the most valued items and distributing is the most effective. Duping is considered cheating and can get you banned on multiplayer servers. Most of them usually patched these dupes using Third-Party plugins.