When an item/object is dropped into a hopper, randomly the item/object it's duplicated x2 to x4 in stacks of up to 64. The bug usually occurs more times if the items/objects are droped in stacks of 64.
Image: Two hoppers making a "pipe" duplicates the stack. 64 to 64x2.
Edit: Last 2 images show a stack going unlimited, beacuse he duplicates himself every time.