ver:utility-belt-neoforge-2.1.1+1.20.6.jar
Neoforge: 20.6.75

When I right-click to add tools to the belt, like you do with a bundle, the tool is duplicated in the belt.

And if the belt is full and I try to right-click to remove the items, it will delete two items.

Weird thing: this does not happen in creative mode

Thanks for reporting. I started looking into this but decided I'll just work on it tomorrow, because I think the fix should involve some refactoring to make sure stuff like this can't happen

Fixed, might not release for a while though as I want to do some intensive play testing to find any other issues