Duplicating

Hey Minecraft! I wanted to report an issue that has been annoying me for quite some time... DUPLICATING! I own a realm where me and my friends play and one of them has figured out how to duplicate watching a YT video. I want you to fix this PROBLEM! They have overrun our realm with diamonds and gold. Minecraft just isn't fun anymore. Here is a video showing off the glitch... https://www.youtube.com/watch?v=-CnsfvmWoMk