i just found a legit way to duplicate! it doesnt require mods or glitches! it may be kind of hard to accomplish though. first you need a diamond pickaxe. then mine alot and get enough diamonds to craft a diamond block. get up enough XP to get up to at least level 20 and enchant your diamond pickaxe (an iron pickaxe will work for this part also) and try to get the fortune enchantment. keep trying until you get it. dont get frustrated because this part may take the longest time. once you get it place down the diamond block (or other block you want to duplicate) and mine it with your pickaxe enchanted with fortune. keep doing this and you should have lots of that one block. if its a block made of an ore like diamond, when your ready put the block onto the crafting table to get the ores back and your finished! if you have any other ways to duplicate (without mods, but glitches are fine) feel free to comment. oh and i reccomend enchanting plus mod to enchant your pickaxe(s) but you dont have to use it.