The "ID in use" error is simply the server telling you that according to it, you're still logged in.
Now, reguardless if you are or not is not important. There is usually two ways this is fixed.
1.) Let the server time you out.
Have you ever noticed how if you leave your game to sit for too long, the server will say that it has logged you out and will return you to the main screen?
This is simply what you'll be doing here - waiting patiently for the server to log you out.
Just wait an hour or so. This should give the server enough time to log you out due to inactivity and allow you to log in again.
2.) Click the "Disconnect current Session" dialogue box
Personally, this is the option that should be taken first.
Usually if there is an ID in use error, a dialogue box will pop up asking if you would like to discontinue the current session. If you're more than lucky to get this option, go ahead and click it.
This is usually the first and only thing you need to do in this situation but if #2 fails, just go with #1.