Relevant mods/loader used (full modlist in video):
Forge version 43.4.2
IC2 Classic version 1.19.2-2.1.1
Sophisticated Core 1.19.2-0.6.4.730
Sophisticated Backpacks 1.19.2-3.20.2.1035
Sophisticated Storage 1.19.2-0.9.7.765

IC2 Classic features a few items that have an inventory that can be accessed from interacting with it within the player's inventory.
Some examples include: Upgrade Containers and Toolboxes.

Sophisticated Storage/Backpacks allows their containers to be upgraded to increase the number of items possible per slot, going above 64 per slot (or above 1 for unstackable items).
This however, allows for a dupe glitch when attempting to open the inventory of the aforementioned inventory-accessible items while they are stacked.

Since they can be still accessed while in this stacked form, any items inserted into this inventory will apply to the entire stack, allowing the stack to be unstacked and then retrieving the items from each individual inventory-accessible item.

And my own two cents, just disabling the inventory-access feature when accessing any GUI that isn't purely the player's inventory GUI is a reasonable fix, as this issue will occur with any mod that can stack unstackable items.
This fix would also patch a similar issue that makes the inventory-accessible items enter a bugged state where they can't be accessed at all.

Video demonstrating the dupe: https://youtu.be/B55odQ86LOE