You can use the furnace dupe it does work. It can take a while even if you are good at this method. are you holding down the action button and switching to the sword without letting go of the action button?
Put what you want to dupe in the ingredient section of the furnace, then destroy it. When it's almost broken, switch to your hand, it will destroy then re-appear. Hold a on the item in the furnace and eventually it may pop out as two, pick one up. (You may have to move some stuff in your inventory to pick it up).
It will be a glitched item, hold x down and you can make unlimited stacks.
I used this method to build a couple of buildings and get all the paper for the currency in my RPG server.
This should work, if you don't get it on the first try, don't give up. It took me several tries. (BTW doing this kills your furnace, no matter if it worked or not. So you better have a **** ton of furnaces if you're bad at this.)
I find just wiring the furnace to TNT is the most effective way....... Flip a lever/button and open the furnace...... Make sure to have the TNT inside a small obsidian room with the furnace next to it....... save first!!!! That way if everything gets screwed you can just reload the game... Look up Duping by yourself on youtube.. Lots of ways to do it.
This didnt work for me at first but then I tried it with a Chest and it worked flawlessly. It doesnt seem to work with more than two tems at a time. Dupe two items and wait for the glitched items to disappear before trying again. Hope this heps!