I see there are default commands already assigned to the list, however, if I add more to the list they are not getting blocked eg /kit so now the players go and duel and kit up during the duel.
Blacklist commands are enabled
Have you tried it yourself to see if it works? Right now it's case sensitive, so that could be the issue. The version below fixes that. Let me know if it solves the issue for you, or not.
In response to nivex ():
Yes, I have added few commands like, "/tpb /buy /kit /tpr /tpa /recall /spawn" and everyone still can. I'll try the new one you gave and let you know. I kept all the commands lower case. The same as if you would have used them in chat
It loaded version 1.21 however it didn't disable the commands thank you for looking into it
"Allow Announcement": true,
"Auto Create Dueling Zone If Zone Does Not Exist": true,
"Auto Enable Dueling If Zone(s) Exist": true,
"Blacklist Commands": true,
"Blacklisted Chat Commands": [