Duplication Glitch

I'm making a vending Machine where you use emeralds to get items from the machine, I have 3 emeralds in my inventory and when I click the button in GUI to give me a item it does remove 1 emerald but gives me 3 more and places it in my hotbar. I have it set up so when you right click on the Machine it opens a GUI where it shows food and drinks (I Currently have Drinks set up at this time) then when you click one of the buttons it takes you to another GUI where you can chose a drink and when I click on one of them that's ween the glitch happens and I still get the item.

https://imgur.com/gallery/fBVZwTe