First, you need a buddy, or a pal. (Doesn't work if your a lone wolf)

https://steamuserimages-a.akamaihd.net/ugc/1862800643596966327/45644ECE97B6261367E618BFEE94F07074C9C8CD/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Second, get materials and make a crate, you will need 3 logs and 4 pieces of raw iron. along with two pieces of coal. Craft these into logs and cooked iron. Then, using the materials, create a crate.

https://steamuserimages-a.akamaihd.net/ugc/1862800643596966633/97E05A8E9E34461B9F755660152CC576857A5FB5/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Third, with the crate, place it in a safe location and get your buddy. Put a desired target of duplication into it.

https://steamuserimages-a.akamaihd.net/ugc/1862800643596972165/B917179FA96B165C318B4DB8488AB850D6F670CC/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Step four, you and you're friend start to click said desired item multiple times, preferably very fast. In this case I'm using a pistol. For step four, I recommend using a auto clicker or macro.

https://steamuserimages-a.akamaihd.net/ugc/1862800643596972734/C277A3C8FC6020A26A5AFD04FDEFF451E0878BFA/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Fifth, if you do it correctly, the item should have been multiplied by two. works with all stacks and all items, even if its a gun, AD rocket, grenades, blood stone, diamonds, etc.

https://steamuserimages-a.akamaihd.net/ugc/1862800643596973322/C391E60028CD64C46CD7994EE3D0E871616A6B3A/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false