What should(n't) happen:

I guess hoppers shouldn't duplicate (multiply is a better word) items.

What did happen:

Whilst using a grid of hoppers (for use in such things as chicken coops). Dropping one stacks worth of an item (in separate units) onto the grid, once it had filtered to the lowest state hopper (central one) there would be around 10-12 stacks altogether in that hopper and the second lowest state hoppers.

How to reproduce:

1. Create a planar (perhaps 3D (though I didn't)) grid of hoppers
2. Drop a small number of individual and identical items (i.e. seeds) onto the grid
3. The lowest state hoppers will have more of the item than was dropped onto the grid.