Sometimes, some people want to duplicate items, some people don't, but I'll tell you anyways. Now you will need to have access to the Space Anomaly and internet. You'll need someone else to do it, BUT you don't have to join a group or anything, just a random person in the Anomaly to give an item to.
Here's the link: https://www.youtube.com/watch?v=6YXcjCyFOZg
It's probably quicker for me to explain it though.
Land at the Space Anomaly
Put the items you want to dupe into your starship inventory
Autosave by getting out of your ship, if you are out of your ship go back in it then out of it again.
Put the items you want to dupe into your exosuit inventory, you can also use the exosuit high capacity inventory.
Give any of your items to someone, if it's from your exosuit inventory it will be gone permanently, but if it's from the starship it will come back once you do the dupe.
Now you should have the items in your exosuit as well as in the starship, so they have been doubled.
Repeat as much as you like.
A lot of people have full inventories, so it might take some time to find someone with space to receive an item, if you can't find anyone without a full inventory, giving a bit of a common element should work, as they probably have a stack of it that isn't full.
It was good for duping upgrade modules, I ended up with way more than I needed so I gave the extras away. Now I'll be doing Stasis Devices, richness, here I come!