If you shift + right click using the break spell on a container it will first open the container then break it. This allows you to take items from said inventory which duplicates them.

Did you actually successfully duplicate an item? Are you on a server or single player? if you are on a server, is this a standard forge server or are you running plugins?

A mod list would also be helpful. Thank you

I am playing on a single player world using the Direwolf20 1.16.5 mod pack. After further testing it I have found that it is not an issue with vanilla minecraft containers. I have only been able to duplicate items so far by using Quantum Storage diamond storage crates. I will be testing other containers as well to see if it is specific to Quantum Storage or if it works with other mods as well.

To clarify This only works with containers that keep their inventory like shulkers.

So the mods that I have been able to successfully duplicate items with using this method are Quantum Storage, Quartz Chests and Krate.

This would be a bug with those mods, they will need to fix their validations on their containers, see a similar issue here: gigabit101/QuantumStorage#239