This kind of error happens when you try to connect to your IRC account and you are using a nickname that is already being used by someone else on that particular network.
Click the 
icon in the error message.
In the Nickname text box, type a new nickname.
If you have registered that nickname within the network you are using, set the password for that nickname. For more information, see Use a nickname password on IRC.
Switch the account off, and then switch it on to try to reconnect to the service.
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