Item Duplication

Hold LeftAlt while clicking on an item, and it will duplicate the items.

Installation

With Mod Manager

Simply open the mod manager (if you don't have it install it here), select Item_Duplication by HarriXS, then Download. If prompted to download with dependencies, select Yes.

Then just click Start modded, and the game will run with the mod installed.

Manually

Install BepInEx here

Then drag DSP_ItemDuplicator.dll into steamapps/common/Dyson Sphere Program/BepInEx/plugins