Saves the username of each joined user and prohibits impersonations.
Does require your server to have a database, you may need to rebuild the plugin when changing the database if the driver is not recognized, simply add your database dependency to gradle!
Simply copy both files UniqueUsernamesPlugin.jar and unique-usernames-config.json into your <server-folder>/config/mods directory
In unique-usernames-config.json the settings for your database connection must be specified.
Use freeusername <username> in the server console to free up a username which has already been taken, this kicks the player with the given name from the server if online and deletes the entry from the database.