Duplicate stackable items in hopperclock while un/loading chunks

When making a clock with 2 hoppers and frequently load/unload the chunk, it may happen, that items inside the hoppers are duplicating. I have the impression, that it's duplicating items, what are currently transfered and depending on how the state of transfer is while loading or unloading the chunk, where the hopperclock is inside.

It doesn't seem to work with singleslot items, but i.e. with diamonds and/or diamondblocks, it's working well.