Hi, one of my players found a duplication bug with dung and a composter

This plugin can't cause item duplication by itself. All it does is take the items out of the container and then add them back.

For what it's worth, this issue has been reported before, and the cause turned out to be due to a dung splitter plugin which had logic to create items under some circumstances, leading to the item duplication. Please check any plugin with "Splitter" in the name.