Please, move chat-prefix to lang file so we could change it for different localizations.

        protected string Prefix { get; } = "[ <color=#406B35>Duelist</color> ]: ";

                ["Prefix"] = "[ <color=#406B35>Duelist</color> ]: ",

Also consider to change some prints (maybe all?) to use this prefix. Now you are using it only in one message so it makes it less useful than it could be.