When a stack of an item enters a hopper and space exists only for part of the stack, the stack is bounced out and duplicated, creating the stack in the world, and a stack inside of the hopper.

To replicate:

1) Configure hopper as shown

2) Drop a stack AS ONE, so that it is a single stack in the world, ontop of the hopper

3) Shift click to remove half a stack of duped item at a time.