I was trying out something with the hopper to see if there was a limit to how far it could go but forgot a chest under it. As I flew down I clicked on the hopper and saw it had cloned my items by two. After throwing it in a few more times (one by one) I saw it did it just about everytime. Then I hit ctrl+q to see what would happen and it cloned the entire stack. When I threw it in it was a 64 stack and when I went and looked it was two 64 stacks. The issue still happened when I put a chest under the hopper. I lessened the amount of connected hoppers to the point where there was only one hopper above the chest and the issue continued. The issue continues to happen when I switch to survival mode.