If you make a setup like this
An put at least 3 items in the top chest it will duplicate if you put one stack in this is what you get
This is a big issue and needs to be fixed!

I saw xisuma's video and it should hopefully be fixed in 3.0.0. Check the 3.0.0 update and let me know.

I can confirm it doesn't do this anymore for me in 1.10.2 3.0.0, but I'd like to have another person confirm.

Edit: You can do this just by throwing items in a setup like this:

Since I haven't received back a reply within 5 days, I'm going to close the issue as I haven't been given any notice as to whether or not it still exists in 3.0.0+.

Feel free to re-open if you happen to get a change to provide some more info :D.