When I made a new world and opened it, I started off with the items that were already in my hotbar. Then I changed the items in my hotbar. After I changed the items, I clicked on the armor tab and then I had 64 items of everything on my hotbar. I checked my inventory and there was 64 of everything. The only way to get rid of it was to empty it in a chest and this thing only happens in creative mode. Also, you only get 64 of every item when it's a new item in your hotbar but after you get rid of 63 of them, the items in the hotbar won't duplicate again.