Hi, server says this bug "[ItemJoin] Failed to Give you 1 items, you are not allowed to overwrite items!"
Please help?

So the easiest solution is to just enable overwrites in the items.yml at the top of the items.yml
Its called items-Overwrite set that to true.

EDIT: Closing due to inactivity, a new official release of ItemJoin has just been approved.
You can now download it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=451699