Hoppers duplicating items

I'm using this guy's dropper-based item elevator design: https://www.youtube.com/watch?v=1F0lbtirC6g and apparently the single item that's put into the hoppers that trade the item back and forth (to power-unpower the comparator that powers-unpowers the bottom dropper) duplicates somewhat frequently. I'm not sure how frequently it duplicates or when (I assume it's when the chunks are un/re-loaded) but it appears to happen with some frequency as I've left it running for around a week and it's gone from having 1 item in the hoppers to having 4 stacks of items in the hoppers (which, in case it isn't obvious, completely breaks the elevator). I'm really not sure how to duplicate this, and I don't think it's affected versions before this as I've been using it for quite some time now (1.10ish). Your best bet to duplicate it would probably be to 1. Make the elevator, 2. Add a lot of items into the input, 3. Experiment with leaving the chunks and returning, waiting for a while, etc.