Hoppers duplicating items

I'm working on a project that requires the use of Hopper Clocks. So, as normal, I leave the chunk area to go somewhere else to work on it. But, whenever I come back, the items within the hoppers have duplicated! This stops the hopper clocks from working! ;-; Is there any way you can fix this bug?

Steps to Reproduce:
1. Create a hopper clock (Place to hoppers facing each other)
2. Leave the chunks that the hoppers are in. Make sure these aren't the spawn chunks!
3. Try to stay outside the chunks for a couple of minutes. It may take a few tries, but when you come back the items in the hoppers will have duplicated