Duplication with Stars

All you need to do is just to throw the item you want to duplicate and the nether star on top of obsidian and let magic do the rest...

Information is available on the wiki.

In this difficulty, unlike others, you won't need lightning.
All you have to do is to place down a hopper and disable the hopper with redstone. You can do this by placing a lever next to the hopper and turning the lever on.

In the hopper, put some nether stars in the third slot, and in the second slot, put some amethyst shards.
The amount of amethyst shards must be equal or higher than four times the number of nether stars. In other words, you will need (4 * the number of nether stars) amethysts. You can change the amount of amethysts required for each nether stars in the configurations.

Now, throw a single copper block on top of the hopper. If you do this, the copper block will disappear, but you will end up with a duplication star: (Ignore the "Hold Ctrl to show more NBT", and the redstone lamp is optional)

So with this information, You will now need to obtain 2 duplication stars (The amount can be changed in the config).
After that, put those duplication stars in the third slot of the hopper you used earlier to make duplication stars.
Now, put the items you want to duplicate in the first and the second slot of the hopper.

Now, make a lightning strike happen near the hopper again. (Just like the last time)
And the items will duplicate!

All you need to do is to place down a hopper. Optionally, and recommendably, place down a lightning rod next to it.

And in that hopper, put some nether stars in the third slot, and in the second slot, put some amethyst shards.
The amount of amethyst shards must be equal or higher than four times the number of nether stars. In other words, you will need (4 * the number of nether stars) amethysts. You can change the amount of amethysts required for each nether stars in the configurations.

Now, make a lightning strike happen near the hopper. You can do this by placing down a lightning rod next to the hopper and throwing a channeling trident on the rod. If you do this, you will end up with a duplication star: (Ignore the "Hold Ctrl to show more NBT")

So with this information, You will now need to obtain 2 duplication stars (The amount can be changed in the config).
After that, put those duplication stars in the third slot of the hopper you used earlier to make duplication stars.
Now, put the items you want to duplicate in the first and the second slot of the hopper.

Now, make a lightning strike happen near the hopper again (Just like the last time), and the items will duplicate!

You will need to place down a hopper, followed by 4 lightning rods next to it and 9 copper blocks below. It should look something like this:

and in the hopper, put some nether stars in the third slot, and in the second slot, put some amethyst shards. The amount of amethyst shards must be equal or higher than four times the number of nether stars. In other words, you will need (4 * the number of nether stars) amethysts. You can change the amount of amethysts required for each nether stars in the configurations.

Now, make a lightning strike happen on one of the lightning rods. You can do this by throwing a channeling trident on the rod. If you do this, you will end up with a duplication star: (Ignore the "Hold Ctrl to show more NBT")

So with this information, You will now need to obtain 2 duplication stars (The amount can be changed in the config).
After that, put those duplication stars in the third slot of the hopper in the setup you used earlier to make duplication stars.
Now, put the items you want to duplicate in the first and the second slot of the hopper.

Now, make a lightning strike happen near the hopper again (Just like the last time), and the items will duplicate!