An item entity placed atop a half slab will not be removed when the hopper below the half slab detects the item entity and "copies" the item into its inventory. It will create about 3 of the item per second indefinitely until the entity is no longer within the block above the hopper.

Sorry if this is a duplicate report. I searched for "duplication half slabs" and couldn't find a similar report, they all seem to be related to random duplication of an extra item here or there, since sitting on the lip was fixed with 13w01b.