I've reproduced this twice now, (unfortunately I lost a nicely enchanted weapon) by using a tool on the utility belt. I swap from one of my hotbar items to the tool in my belt (toggle or hold, both do it) and as soon as I start using it for mining/digging etc, it duplicates itself onto the hotbar slot of the item I was using before moving to the belt tool. This both erases the item on the hotbar as well as creates a duplicate of the item I'm using in the belt.

What Minecraft/Utility Belt version are you using?

Oh, I did not read the issue title properly...will look into this now

This is my attempt to reproduce the issue: https://streamable.com/99jspp

Not sure how to reproduce it