The duplication glitch I am talking about allows a player to duplicate any block by using a sticky piston, redstone clock, and any block. One time I was playing on a PvP server, and some guy duplicated diamond ores he found and duplicated again and again, and then he had all diamond armor and sword. Everytime I killed him (very rare) he just spawned back in, and then came rushing out of his base with diamonds. So I raided it and saw his duplicator. I think the problem is that the piston doesn't respond to the block fast enough, so it creates a small gap to place any other block. But really, Minecraft THINKS that the block you placed is the block that was there before, so it creates another one.