LAN item duplication - deleting dup deleted original too

Local Multiplayer Duplication Glitch. I was playing on my world with my little brother and, all of the sudden, he tells me that he had 2 diamond axes! I was playing on Toolbox for MCPE, he was on MCPE. So, I then delete the axe with my mod and it was gone. Then we decided to go mining. THE PICKAXE HE HAD WAS GONE. I don't know what happened or how it did, but I'm OCD and this irritates me more than ever.