Port is Duplicated Error

I don’t know how to fix this. My friend what trying to port forward but he gets the Duplicated port error. How do I fix this so I can port forward it.

In most cases you get this error message if the forwarding port was already forwarded by another forwarding rule.

If you host the Server at Home please take have a look into this Thread: [Solved] Hosting a TS3 Server at home - #20 by Rikku